Xorns in stone outside Sokobon and other special levels

Xorns in stone outside Sokobon and other special levels
First reported 12/13/2003, I think, but my archives contain more recent reports
too.  Special level specs like NON_PASSWALL and NON_DIGGABLE only apply
to the map sections for which they are specified.  So, on special levels of
Sokobon, the large surrounding area is stone, but have no special flags set.
So, it was possible to teleport a Xorn and have it appear in this outer area.

Addressed this by checking the perimeter of the map(s) and if all are
nondiggable and nonpassable, propagate this to the surrounding stone.
This appears to be the intent on such levels, so there is no need to
force the affected special levels to explicitly specify this.