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
Copter: change CH7/CH8 pre-arm check string
Copter: 3 positions switch support for CH7 and CH8 - new 3 positions SIMPLE / SUPERSIMPLE mode

3 positions modes must use mode number >= 100
pre-arm check correction for CH7 - CH8 check
Copter: generalize 3 pos switch for all ch7/ch8 options
Copter: remove unused ap.gps_status flag
Copter: resolve compiler warning re unused variable
Copter: bug fix to look_ahead_yaw

Fix received from David Dewey
Was incorrectly using the direction instead of heading when deciding
whether to point forward or not.  Reduced YAW_LOOK_AHEAD_MIN_SPEED to
1m/s because bug meant it was nearly always pointing forward and 10m/s
(the old value) seemed too high
Plane: use new scheduler load_average() function

this also simplifies the main loop
Copter: use the new scheduler load_average() call

this also simplifies the main loop