Add dynamic modules on Posix.

You can now add `DYNAMIC` as an option to `px4_add_module`, which will
cause that module to no longer be compiled into the px4 executable, but
instead produce a separate shared library file, which can be loaded and
executed with the new `dyn` command:

    pxh> dyn ./hello.px4mod start

This will load the shared object file `hello.px4mod` if it wasn't
already loaded, and execute its main function with the given arguments.