(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/saveload.cpp

@@ -28,7 +28,7 @@
 #include "variables.h"
 #include <setjmp.h>
 
-extern const uint16 SAVEGAME_VERSION = 50;
+extern const uint16 SAVEGAME_VERSION = 51;
 uint16 _sl_version;       ///< the major savegame version identifier
 byte   _sl_minor_version; ///< the minor savegame version, DO NOT USE!