(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

network_server.c

@@ -5,6 +5,7 @@
 #include "string.h"
 #include "strings.h"
 #include "network_data.h"
+#include "train.h"
 
 #ifdef ENABLE_NETWORK
 
@@ -1271,7 +1272,7 @@ void NetworkPopulateCompanyInfo(void)
 		if (v->owner < MAX_PLAYERS)
 			switch (v->type) {
 				case VEH_Train:
-					if (v->subtype == TS_Front_Engine)
+					if (IsFrontEngine(v))
 						_network_player_info[v->owner].num_vehicle[0]++;
 					break;
 				case VEH_Road: