(svn r3362) Fix issue with loading old (TTD) savegames:

- move call to convert from old to new train subtypes.
 - ensure AI is started for AI players.

vehicle.h

@@ -317,6 +317,7 @@ int32 GetTrainRunningCost(const Vehicle *v);
 int CheckTrainStoppedInDepot(const Vehicle *v);
 
 bool VehicleNeedsService(const Vehicle *v);
+void ConvertOldMultiheadToNew(void);
 
 typedef struct GetNewVehiclePosResult {
 	int x,y;