Add function to get detected channels in PPM stream

This function will be used by rx.c to check puls duration for all
channels.
This will be used for failsafe detection feature.
Failsafe rewrite completed (benchtested)

Failsafe can now be activated with an RC switch or when the RC link
fails.
Arming is blocked when failsafe is actived with a switch.
All functionality should be field tested now.