Copter: add flags for upper and lower throttle limits

Freeze desired altitude when motors hit a limit
Copter: reset loiter target when landed
Copter: bug fix for take-off in loiter
Copter: clear thr accel I term on take-off
Copter: allow arming in AltHold and Loiter if landed
Copter: landing check changes
Copter Motors: by default spin motors at 65 when armed
Copter: turn off stability patch when landed in Loiter
Copter: remove landing checks in loiter controller
Copter: deactivate accel throttle controller when landed
Copter: set landing_complete flag to false on start-up
Copter: minor format change to exit_mission function