(svn r8978) -Feature: Rewrite of transfer system.

This major feature-fix keeps track of the value of transfers offloaded at a station, and hands on to a loading vehicle a credit note proportionate to the cargo loaded. The transferring vehicle is paid only for the distance it has travelled. eg. B-C in a transfer chain A-B-C-D.
When the final cargo is delivered, these credit notes are cashed, and the final vehicle is paid for A-D minus any credit notes it is carrying. The company bank balance increases by the value of the A-D route.
You still need to set up a profitable route; it is still easy to make a loss on transfers by bad planning. :)

src/station.h

@@ -15,6 +15,7 @@ static const StationID INVALID_STATION = 0xFFFF;
 typedef struct GoodsEntry {
 	GoodsEntry() :
 		waiting_acceptance(0),
+		unload_pending(0),
 		days_since_pickup(0),
 		rating(175),
 		enroute_from(INVALID_STATION),
@@ -25,6 +26,7 @@ typedef struct GoodsEntry {
 	{}
 
 	uint16 waiting_acceptance;
+	uint16 unload_pending;        ///< records how much cargo is awaiting transfer during gradual loading to allow correct fee calc
 	byte days_since_pickup;
 	byte rating;
 	StationID enroute_from;