Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.
Fix the handling of FIONREAD/FIONWRITE; thanks Tridge.
Fix a typo that caused PWM_SERVO_GET ioctls to fail on the FMU PWM outputs.
Merge branch 'master' of github.com:PX4/Firmware into fixedwing_io_mixing