(svn r853) -Feature: Implement improved vehicle loading algorithm

It's not FIFO loading, but does the right thing in the common case:
If a vehicle is empty and another vehicle is already loading the same cargo at this station then the vehicle waits.

This is an reworked version of [ 1072211 ] submitted by Hackykid, thanks!

variables.h

@@ -87,6 +87,7 @@ typedef struct Patches {
 	bool mammoth_trains;		// allow very long trains
 	bool join_stations;			// allow joining of train stations
 	bool full_load_any;			// new full load calculation, any cargo must be full
+	bool improved_load;			// improved loading algorithm
 	byte station_spread;		// amount a station may spread
 	bool inflation;					// disable inflation
 	bool selectgoods;       // only send the goods to station if a train has been there