Codechange: remove the special station/vehicle code from SaveLoad

With the new SLEG_STRUCT it is much easier to embed a struct
in a struct, where the sub-struct has limitations on when it is
being used.
This makes both the code easier to read (less magic) and avoids
the SaveLoad needing to know all these things about Stations
and Vehicles.

src/cargopacket.h

@@ -305,8 +305,8 @@ public:
 	friend class StationCargoList;
 	/** The super class ought to know what it's doing. */
 	friend class CargoList<VehicleCargoList, CargoPacketList>;
-	/** The vehicles have a cargo list (and we want that saved). */
-	friend SaveLoadTable GetVehicleDescription(VehicleType vt);
+	/* So we can use private/protected variables in the saveload code */
+	friend class SlVehicleCommon;
 
 	friend class CargoShift;
 	friend class CargoTransfer;