Initial implementation of application access to the PX4IO relays.
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.
Ignore more.
Merge pull request #123 from PX4/#106-px4io-relays

Initial implementation of application access to the PX4IO relays.
Merge branch 'master' into #111-px4io-integrated-mixing

Fix a botched merge for #106 at the same time.