(svn r3138) -Fix: [clone vehicles] fixed assert when it was possible to allocate some vehicles to clone a train, but not for all cars

Now it gives "too many vehicles" error message instead
	To make this work, AllocateVehicles() needed to be moved to vehicle.c (from aircraft_cmd.c) and made non-static

vehicle.h

@@ -274,6 +274,7 @@ typedef void *VehicleFromPosProc(Vehicle *v, void *data);
 
 void VehicleServiceInDepot(Vehicle *v);
 Vehicle *AllocateVehicle(void);
+bool AllocateVehicles(Vehicle **vl, int num);
 Vehicle *ForceAllocateVehicle(void);
 Vehicle *ForceAllocateSpecialVehicle(void);
 void UpdateVehiclePosHash(Vehicle *v, int x, int y);