Plane: altitude for do_set_home() is in meters
Plane: use mission.resume() not mission.start()
AP_Mission: added truncate() function

Used when loading a mission while flying

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Plane: fixed relative altitudes and resuming mission

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Plane: only activate mission callbacks in AUTO mode
Plane: fixed build (forward declarations)