ArduPPM, possibility for throttle low on single channel errors

Changes to ArduPPM:

Two new defines added: 

_THROTTLE_LOW_FAILSAFE_INDICATION: Throttle low indication and thus APM fail-safe triggering when a single channel error is detected.

_THROTTLE_LOW_RECOVERY_POSSIBLE: Throttle low is reset when a disconnected channel comes back and full control regained.

By default, the two defines are commented/not active.