Plane: set sys_status motor outputs bit from safety switch
Plane: allow GCS to turn safety switch on/off
Rover: set sys_status motor outputs bit from safety switch
Rover: allow GCS to turn safety switch on/off
GCS_MAVLink: added handle_set_mode() function
Plane: use handle_set_mode()
Copter: use handle_set_mode()
Rover: use handle_set_mode()