Improved RX failure handling on PX4, prevent fly-aways when using SBus

This change emulates a throttle failsafe signal of 800us if the RX signal is lost completely, e.g. on total RX failure or RX disconnect. Additionally, this change resolves #645, as the PX4 low level software currently stops providing any RX telegrams if a failsafe occurrs. Without this change, the result would be a fly-away on any RX failsafe here.

The value of 800us is chosen intentionally, as most users will only test their FS settings by powering down their TX and set their FS_THR_VALUE accordingly. We have to be sure to be way below.

Tested with SBus (Futaba R6108SB, FrSKY TFR8SB) and PPM (Simprop Gigascan 9 FASST) on PX4 so far.