(svn r3218) -Feature: Multiheaded train engines will now stay in the same train

This means that any user attempt to remove a rear engine will tell the user to move the front engine instead
	This fixes the assert when moving multiheaded engines (introduced in r3144)
	Note: to make old savegames use this feature, some engines might be turned around in order to link engines in pairs

-Codechange: train subtype is now a bitmask
	This allows fast access to info like if it is a wagon or engine and if it is in front and so on
	Note: savegame version bump

station_cmd.c

@@ -26,6 +26,7 @@
 #include "sprite.h"
 #include "depot.h"
 #include "pbs.h"
+#include "train.h"
 
 enum {
 	/* Max stations: 64000 (64 * 1000) */
@@ -2255,7 +2256,7 @@ static uint32 VehicleEnter_Station(Vehicle *v, TileIndex tile, int x, int y)
 	byte dir;
 
 	if (v->type == VEH_Train) {
-		if (IS_BYTE_INSIDE(_m[tile].m5, 0, 8) && v->subtype == TS_Front_Engine &&
+		if (IS_BYTE_INSIDE(_m[tile].m5, 0, 8) && IsFrontEngine(v) &&
 				!IsCompatibleTrainStationTile(tile + TileOffsByDir(v->direction >> 1), tile)) {
 
 			station_id = _m[tile].m2;