Stairwells : removed L1,L2,L3 shapes (they only occur extremely rarely)
Stairwell : worked on computing the necessary points for making the brushes.
Stairwells : actually build it, compute the brushes for the floor, ceiling
and side walls.  It is working! :)
Stairwells : added logic to decide stair height, and to properly set the
exit height for the next room.
Stairwells : fixed some stairwells facing the wrong way.
Stairwells : a few fixes.
minor renaming...
Support for "quiet_start" gameplay tweak -- never add any monsters into
the starting room.