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