Motors: add get_motor_mask method

This returns a bitmask of output channels that are used by the motors
library.  This allows removing the chance of a do-set-servo interfering
with a motor or control surface used to control the vehicle.
MotorsTri: implement get_motor_mask
MotorsSingle: implement get_motor_mask
TradHeli: implement get_motor_mask
MotorsCoax: implement get_motor_mask
MotorsMatrix: implement get_motor_mask
Copter:  pass motor_mask to ServoRelayEvents
Plane: prevent integer underflow with comm_get_txspace()
GCS_MAVLink: prevent integer underflow with comm_get_txspace()
autotest: use run_in_terminal.sh for HIL
Rover: avoid integer underflow in mavlink txspace check
Copter: avoid integer underflow in mavlink txspace check
FollowMe: avoid integer underflow in mavlink txspace check
AntennaTracker: avoid integer underflow in mavlink txspace check
AP_InertialSensor: avoid integer underflow in mavlink txspace check
GCS_MAVLink: avoid integer underflow in mavlink txspace check
Rover: change default throttle slew rate to 100

this will help with NiMH batteries which may brownout with sudden
currents