[rotorcraft] fix MODE_STARTUP

- init nav, guidance and stabilization from autopilot_init
- after init of the above, set the autopilot_mode so it propagates to the guidance submodes.
- also call autopilot_init after the ahrs_init for ahrs_is_aligned check...
Added check if autopilot_motors_on do determine if you want to switch to AP_MODE_FAILSAFE
Merge pull request #473 from flixr/rotorcraft_startup_mode

[rotorcraft] fix MODE_STARTUP