- GetSectorFloorZ/GetSectorCeilingZ tag=0 handling (take 4)

With this patch GetSectorFloorZ and GetSectorCeilingZ consider tag=0 to mean 'any sector' and look up the sector at the coordinates given. tag=0 is useless as it is because the function first gets the sector by tag first and then gets the coordinates, so it only works for single tagged sectors. This adds a generalized way of getting z by x and y at whatever point in the map.