[rotorcraft] add a HOME mode

similar to HOME mode in fixedwing firmware.
[rotorcraft] go to FAILSAFE mode after 1.5*MAX_DIST_FROM_HOME

In failsafe the rotorcraft should perform a landing which makes more sense than KILL.
You can still set KILL_AS_FAILSAFE to kill instead.
[rotorcraft] add UNLOCKED_HOME_MODE
[gcs] connect the 'back to nav' strip button

also remove some warnings since it seems to be more confusing than
inactive buttons in the strip
[failsafe] settable failsafe mode

- failsafe mode when aircraft is really to far can be set
- descent speed in failsafe mode can be set
- choose more realistic values for max_dist_from_home in default FP
[rotorcraft] add RC_LOST_MODE and stricter rc loss check
Merge pull request #562 from paparazzi/rotorcraft_home_mode

[rotorcraft] add a HOME mode

similar to the fixedwing home mode:
- will go into HOME mode if too_far_from_home
- with <define name="UNLOCKED_HOME_MODE" value="TRUE"/> you can get out of HOME mode if you have valid RC and are not too_far_from_home
- if you get really too far from HOME (FAILSAFE_MODE_DISTANCE with default 1.5 * MAX_DIST_FROM_HOME)
  - go into FAILSAFE_MODE_TOO_FAR_FROM_HOME (defaults to AP_MODE_FAILSAFE)

- FAILSAFE descent speed is configurable with FAILSAFE_DESCENT_SPEED (default now 1.5 m/s)
- RC_LOST_MODE now configurable (default is same as before: AP_MODE_FAILSAFE)