Use the right constraint for the output mixer; we might end up wanting more virtual control channels.

Harmonized PPM, S.BUS and DSM input (order: first preference S.Bus, then DSM, then PPM, first available and valid source is chosen), tested with FMU, valid channel inputs

 - Use a separate thread for handing R/C inputs and outputs.
 - Remove all PX4IO R/C receiver configuration; it's all automatic now.
 - Rework the main loop, dedicate it to PX4FMU communications after startup.
 - Fix several issues in the px4io driver that would cause a crash if PX4IO was not responding.

 - enable USART3 for S.bus
 - reduce the scheduling tick to 1ms
 - disable RR scheduling
 - reduce stdio buffer sizes (not used)

Possible fix for #78 - increase the wait timeout for discard when flashing PX4IO. It's not clear this solves the issue, but I can't reproduce it with this added.

Raised COM port number for windows from 18 to 32

Improved termios support for the STM32 UART driver.  Also add a little more termios processing to the generic serial layer.  Implement FIONREAD.