(svn r1817) -Codechange: Moved depot-functions to depot.c

-Codechange: Added wrappers around depot-access (GetDepot no exists)
-Codechange: Made depot-functions a bit more logic (no longer 
GetDepotByTile crashes your game when you request it on a non-depot tile)
-Add: made depots dynamic (yes, 64k depots are possible now)

station_cmd.c

@@ -18,6 +18,7 @@
 #include "airport.h"
 #include "sprite.h"
 #include "npf.h"
+#include "depot.h"
 
 enum {
 	/* Max stations: 64000 (64 * 1000) */