Plane: make a throttle_cruise of 0 mean always zero throttle

this can be used for parachute landing missions

Thanks to Chris Miser for the suggestion
Plane: don't trigger GCS failsafe if GCS never connected

The we have never received a heartbeat message from the GCS then don't
use the lack of heartbeat to trigger a failsafe event
Plane: Added extra parameter to specify at what speed in auto-takeoff throttle should engage

This adds TKOFF_THR_MINSPD in m/s
Plane: added TKOFF_THR_MINACC option

this is used for triggering auto takeoff with a hand launched plane