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
This resolves issue #880 in which if a GPS failsafe occurred while the
vehicle was already in LAND mode, the LAND would continue to be GPS
controlled LAND instead of pilot-controlled LAND