(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_cmd.cpp

@@ -2764,6 +2764,7 @@ static const SaveLoad _station_desc[] = {
 
 static const SaveLoad _goods_desc[] = {
 	    SLE_VAR(GoodsEntry, waiting_acceptance, SLE_UINT16),
+	SLE_CONDVAR(GoodsEntry, unload_pending,     SLE_UINT16,                51, SL_MAX_VERSION),
 	    SLE_VAR(GoodsEntry, days_since_pickup,  SLE_UINT8),
 	    SLE_VAR(GoodsEntry, rating,             SLE_UINT8),
 	SLE_CONDVAR(GoodsEntry, enroute_from,       SLE_FILE_U8 | SLE_VAR_U16,  0, 6),