(svn r12230) -Codechange: [autoreplace] made a function to detect if a vehicle needs autorenewing

This will remove duplicated code and ensure that the check is consistent

src/aircraft_cmd.cpp

@@ -1574,7 +1574,7 @@ static inline bool CheckSendAircraftToHangarForReplacement(const Vehicle *v)
 		/* There is no autoreplace assigned to this EngineID so we will set it to renew to the same type if needed */
 		new_engine = v->engine_type;
 
-		if(!p->engine_renew || (v->age - v->max_age) < p->engine_renew_months * 30) {
+		if (!v->NeedsAutorenewing(p)) {
 			/* No need to replace the aircraft */
 			return false;
 		}