does_wall_exist() returns TRUE if unknown
This is to allow the flood fill algorithm to be able to solve an incomplete MazeMap.
flood fill now runs .._pull_neighbor()
Since there was a lot of duplicated code for each direction, I thought it would make more sense if this was its own function.  While there is a performance hit of creating another function, it reduces overall code size and will make it easier to modify this portion of the code later on.