Plane: set fence manual recovery on mode change

ArduPlane/mode.cpp

@@ -101,6 +101,13 @@ bool Mode::enter()
 
         // update RC failsafe, as mode change may have necessitated changing the failsafe throttle
         plane.control_failsafe();
+
+#if AP_FENCE_ENABLED
+        // pilot requested flight mode change during a fence breach indicates pilot is attempting to manually recover
+        // this flight mode change could be automatic (i.e. fence, battery, GPS or GCS failsafe)
+        // but it should be harmless to disable the fence temporarily in these situations as well
+        plane.fence.manual_recovery_start();
+#endif
     }
 
     return enter_result;