add crash detection, allow disengage via param CRASH_DETECT
improved is_flying behavior
take off, landing and hard-landing improvements
add stillness check to is_flying and log it
minimum airspeed is determined ARSPD_FBW_MIN*0.75
Param 1 of CONTINUE_AND_CHANGE_ALT now denotes which direction the
user expects the plane to travel when changing altitude:
0 = no expectation, command completes when within 5 m of altitude.
1 = climb expected, command completes at or above altitude.
2 = descent expected, command completes at or below altitude.
Whenever next waypoint is within the loiter radius, maintaining loiter would prevent us from ever pointing toward the next waypoint. Hence for very close waypoints loiter_to_alt becomes verified by the altitude only.
