AP_MotorsHeli: add set_desired_rotor_speed_to_idle

Only used when rsc-mode is throttle curve.
gasser motor ESC is set to min when vehicle is disarmed, idle when
armed, and finally a higher value depending upon the collective pitch
(i.e. swash) position when ch8 input is raised.
TradHeli: set rotor speed to idle when using thr curve