z-axis: shift mounts to under parts

z-axis: shift mounts to under parts

I've been using these upside down the entire time... not sure this was
really worth it afterward (box_frame compatibility is still broken,
and all models still have different coordinate spaces...), but rolling
with it -- eventually want to have all models using the same
coordinate space to make the relationships between them
clearer (having to mentally transform coordinates is a pain).

Also cleaned up clearing of any walls that conflict with the motor and
some positioning.