AC_WPNav: prevent I term buildup during landing

this prevents I term buildup in the XY velocity controller during
landing. This to account for the EKF giving a non-zero horizontal
velocity when we have touched down. The I term buildup in the XY
velocity controller can lead to the attitude error going above the
level for disabling the relax function as the throttle mix is
changed. That results in large motor outputs which can tip over the
vehicle after touchdown.

Thanks to Leonard for the suggestion

libraries/AC_WPNav/AC_Loiter.cpp

@@ -147,6 +147,10 @@ void AC_Loiter::soften_for_landing()
 
     // set target position to current position
     _pos_control.set_xy_target(curr_pos.x, curr_pos.y);
+
+    // also prevent I term build up in xy velocity controller. Note
+    // that this flag is reset on each loop, in run_xy_controller()
+    _pos_control.set_limit_accel_xy();
 }
 
 /// set pilot desired acceleration in centi-degrees