(svn r1834) - Fix: NPF does not check the owner of its target, busses try to enter other players' depots. TODO

- Add: asserts to find the v->u.rail.track == 0 problem.
- Add: IsValidDepot(), IsValidTown(), IsValidSign(), IsValidVehicle(), IsValidStation()
- Add: GetTileOwner(), IsTileOwner()
- Codechange: Replaced IsShipDepotTile(), IsTrainDepotTile(), IsRoadDepotTile() by IsTileDepotType().
- Codechange: typedeffed the MAP_OWNERS as Owner. Should be used as variable type.
- Codechange: Replaced a few uint by TileIndex.

rail_cmd.c

@@ -2260,6 +2260,7 @@ static uint32 VehicleEnter_Track(Vehicle *v, uint tile, int x, int y)
 			if ((v=v->next) != NULL) {
 				v->vehstatus &= ~VS_HIDDEN;
 				v->u.rail.track = _depot_track_mask[dir];
+				assert(v->u.rail.track);
 			}
 		}
 	}