AC_WPNav: add clear_pilot_desired_acceleration

This allows quickly clearing out the pilot desired acceleration for
loiter contoller.
Copter: RTL always land if in radio failsafe

Resolves issue in which vehicle would get stuck at RTL_ALT_FINAL if
failsafe occured.
Also clear out pilot acceleration from loiter controller if failsafe
occurs during the final descent.
Copter: Loiter clears out pilot acceleration when failsafe occurs

This ensures invalid pilot desired accelerations are cleared from the
loiter controller.  This is probably not strictly necessary because the
vehicle should switch out of Loiter and into RTL during failsafe.