Add reverse at waypoint orders.

The train will reverse when its tail is within the waypoint tile.
This is useful for reversing on train en-route, without creating
dedicated reversing sidings or platforms.

src/saveload/vehicle_sl.cpp

@@ -724,6 +724,7 @@ const SaveLoad *GetVehicleDescription(VehicleType vt)
 		SLE_CONDNULL(2, 2, 19),
 		 SLE_CONDVAR(Train, gv_flags,            SLE_UINT16,                 139, SL_MAX_VERSION),
 		SLE_CONDNULL(11, 2, 143), // old reserved space
+		SLE_CONDVAR_X(Train, reverse_distance,    SLE_UINT16,                   0, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_REVERSE_AT_WAYPOINT)),
 
 		     SLE_END()
 	};