AP_Motors: Added MOT_UNSAFE_THR parameter - Throttle setting used to signal that the copter is armed or otherwise unsafe to approach
Copter: Warn user with motors while copter is arming
Copter: Cancel arming if throttle input above cruise_throttle.
Copter: do not spin motors during arming process
Copter: renamed MOT_UNSAFE_THR to MOT_SPIN_ARMED

Removed special purpose spin_unsafe function
renamed _throttle_unsafe to _spin_when_armed and changed to an AP_Int8
Copter: implement safety spin for Tricopters