(svn r3954) - Explicitly update v->first in TrainConsistChanged() if necessary, as this is far faster than brute forcing it later.

- When loading a game, call TrainConsistChanged() for each train head separately before updating images, as v->first is used extensively in GetTrainImage() for custom graphics. This gives a significant speed improvement on loading a game. 
- Rewrite GetFreeUnitNumber() so that only one loop of vehicles is required. Instead a list of used/unused numbers is created and the first unused number is chosen. This significantly improves performance in large games. 
- Improve game-load times. Backport of r3570-3572 from trunk

train_cmd.c

@@ -93,6 +93,9 @@ void TrainConsistChanged(Vehicle* v)
 		const RailVehicleInfo *rvi_u = RailVehInfo(u->engine_type);
 		uint16 veh_len;
 
+		// Update the v->first cache. This is faster than having to brute force it later.
+		if (u->first == NULL) u->first = v;
+
 		// update the 'first engine'
 		u->u.rail.first_engine = (v == u) ? INVALID_VEHICLE : first_engine;