This triggered POLLHUP too early on Mac.

Unlike pipes, unix sockets provide bi-directional
communication with each connected client.

- No need to generate a unique uuid per client anymore.

- The client doesn't have to create its own pipe anymore.

- Since there is no risk of multiple client's writes getting mixed up,
  messages don't need to fit in a single write anymore, removing the
  limit on command length.

- Since the server can detect a connection closing, the client no longer
  needs to handle signals. When the client is killed, the connection is
  automatically closed, which will cause the server to kill the related
  px4 thread.
  Since this does not rely on handling signals and the client sending an
  additional message, this is much more reliable.

- Client is no longer a singleton.

- The protocol is simplified. Standard output is directly written to the
  socket back to the client, without wrapping it in any protocol
  message.

- Because of the simple protocol, one could now even use netcat to run a
  px4 command:

    $ echo hello | netcat -UN /tmp/px4-sock-0

Also removes a few race conditions.

Fixes these invalid format strings:
- A `%d` for a pointer (replaced it by `%p`)
- A 0x%08x (and a 0x%0x8!) for a pointer (replaced by %p)
- 2 cases of `%d` for a `ssize_t` (replaced it by `%zi`)
- 1 case of a %u for an `int` (replaced by %i)
- 3 cases of %d for a `long` (replaced by %ld)
- 19 cases of `%d`, `%i`, `%u` or `%lu` for a `size_t` (replaced it by `%zu`)
- An unused formatting argument (removed it)
- A missing `%d` (added it)
- A missing `%s` (added it)
- 2 cases of `%llu` for a `uint64_t` (replaced it by `"%" PRIu64`)
- 6 cases of giving a string directly as format string (replaced it by `("%s", string)`)
- 2 cases of %*-s, which should probably have been %-*s.
  (Looks like NuttX accepts (the invalid) %*-s, but other platforms don't.)
- A %04x for a `uint32_t` (replaced by "%04" PRIx32)

    - nuttx in PX4/Firmware (31767b6b5aeb4293c2991dbfea6b810e34a34149): https://github.com/PX4/NuttX/commit/999c4357a220d9ea271475e74abd5d8ba080be27
    - nuttx current upstream: https://github.com/PX4/NuttX/commit/166d898c70141d7ddb179b5ead1c5ea726574411
    - Changes: https://github.com/PX4/NuttX/compare/999c4357a220d9ea271475e74abd5d8ba080be27...166d898c70141d7ddb179b5ead1c5ea726574411

166d898 2018-10-18 Daniel Agar - [BACKPORT] Merged in dagar/nuttx/pr-stm32_dma_per_spi (pull request #736)
0a5b1cb 2018-10-17 David Sidrane - [BACKPORT] Lampoo-master-spi-flash (pull request #735)

   This common code can be used byt posix based boars to provide
   the board_identity API.

This uses the systems default shell:
- Ubuntu: dash
- Fedora: bash

Since bash is invoked via /bin/sh, it operates in POSIX mode:
  https://tiswww.case.edu/php/chet/bash/POSIX

- remove '# Ignore the expand_aliases command in zshell.'
  Not needed because the shell operates in POSIX mode
- [[ is bashism -> use [
- autostart_files=( $autostart_file_match )
  is not supported in dash, so use 'ls'
- shellcheck runs the dash flavor, since dash is a minimalistic shell.

Tested on dash & bash.

    - nuttx in PX4/Firmware (1adcfeecc20620f02439093afb85f3151b2e8be1): https://github.com/PX4-NuttX/nuttx/commit/e31e94f5bd3cf68b9ed0f010724ac9122730dd21
    - nuttx current upstream: https://github.com/PX4-NuttX/nuttx/commit/7e3c8e10cd92351b905bc8d0e34e69bccd00dfea
    - Changes: https://github.com/PX4-NuttX/nuttx/compare/e31e94f5bd3cf68b9ed0f010724ac9122730dd21...7e3c8e10cd92351b905bc8d0e34e69bccd00dfea

    7e3c8e1 2018-09-21 Gregory Nutt - [BACKPORT] 510b0f7e07 Correct all ARMv7-M architectures.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.

   This insures the common exception handler will not be
   re-entered. The handler does not support nested interrupts
   and the interrupt stack pointer and context will be overwritten
   resulting in hard to debug hardfaults.

   If all the priorities are equal the NVIC prevents the
   preemption. The startup code defaults all the priorities
   to the same value 128.

   This change safeguards in 2 ways 1) By disabling
   CONFIG_ ARCH_IRQPRIO: up_prioritize_irq cannot be called.
   This will insure that all HW interrupts are at the same
   priority.

   2) By disabling CONFIG_ARCH_HIPRI_INTERRUP, the common
   exception will disable any interrupts during interrupt
   processing.