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