(svn r14683) -Change: only say a engine/vehicle is refittable when it can be refitted to at least two cargo type or when it has subcargos.

NOTE: it is still possible that a vehicle is marked refittable when there is nothing to refit to, e.g. when only one subcargo is available but that cannot be determined without a vehicle chain.

src/engine_func.h

@@ -26,6 +26,7 @@ void LoadCustomEngineNames();
 void DeleteCustomEngineNames();
 
 bool IsEngineBuildable(EngineID engine, VehicleType type, CompanyID company);
+bool IsEngineRefittable(EngineID engine);
 CargoID GetEngineCargoType(EngineID engine);
 void SetCachedEngineCounts();