This feature slightly revs the motors in response to the pilot's input
before takeoff AltHold, Loiter, AutoTune, PosHold and Sport flight modes
pair-programmed with Randy Mackay
This resolves issue #1021 in which LAND mode could descend at the
PILOT_VELZ rate instead of the WPNAV_SPEED_DN
Pilot defined acceleration is used for AltHold, AutoTune , Circle,
Hybrid, Loiter, OF_Loiter and Sport flight modes
Waypoint Nav (ie. AutoPilot) acceleration is used for Auto, Land, RTL