Make WALLIFIED_MAZE into a level flag.

It should now be randomly disabled for a 3rd of Gehennom, to make things
a tad more interesting there. It's also disabled in Baalzebub's lair,
to make things a little more interesting.

Still don't know why the beetle is disappearing.

dat/gehennom.des

@@ -502,8 +502,8 @@ TRAP:"magic", random
 #
 # The Baalzebub level
 #
-MAZE:"baalz",random
-FLAGS: noteleport
+MAZE:"baalz",' '
+FLAGS: noteleport,corrmaze
 GEOMETRY:right,center
 MAP
 -------------------------------------------------