add method to block fallback to mission

failsafe navigation modes can use a flag in mission_result to tell the
commander to not switch back to mission

src/modules/navigator/navigator_mode.cpp

@@ -63,6 +63,9 @@ NavigatorMode::run(bool active) {
 		if (_first_run) {
 			/* first run */
 			_first_run = false;
+			/* Reset stay in failsafe flag */
+			_navigator->get_mission_result()->stay_in_failsafe = true;
+			_navigator->publish_mission_result();
 			on_activation();
 
 		} else {