Refactor Link Refresher cargo mask handling.

Add some separate helper functions.
Move cargo checks out of LinkRefresher::Run.

src/vehicle_base.h

@@ -279,6 +279,8 @@ public:
 	/** We want to 'destruct' the right class. */
 	virtual ~Vehicle();
 
+	uint32 GetLastLoadingStationValidCargoMask() const;
+
 	void BeginLoading();
 	void CancelReservation(StationID next, Station *st);
 	void LeaveStation();