Forward flight motor shutdown script for tailsitters and tiltrotors

This allows to shutdown selected motors to be stopped once in forward flight for efficiency.

Set the motors to shutdown in with the stop_motors variable. Enable and disable the functionality with a RC switch with options 300 (Scripting1).

Motors will automatically be shutdown if forward throttle is lower than the value set in throttle_off_threshold (50% by default) the motors will then be re-enabled if the throttle goes above the value set in throttle_on_threshold (75% by default).

Time for stopped motors to go from throttle value to 0 and 0 back to throttle can be set with slew_down_time and slew_up_time.