Navigator: Use maximum flight altitude to limit missions

This change limits all mission items to the maximum flight altitude. The mission will still be executed and flown,
but the vehicle will never exceed the mission altitude. This ensures the vehicle can always reach the mission
items. Wether or not the entire mission should be rejected if it falls outside of the fenced area is enforced
in the mission feasibility checker function.

src/modules/navigator/enginefailure.cpp

@@ -126,7 +126,9 @@ EngineFailure::set_ef_item()
 	reset_mission_item_reached();
 
 	/* convert mission item to current position setpoint and make it valid */
+	mission_apply_limitation(&_mission_item);
 	mission_item_to_position_setpoint(_mission_item, &pos_sp_triplet->current);
+
 	pos_sp_triplet->next.valid = false;
 
 	_navigator->set_position_setpoint_triplet_updated();