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.

src/mklev.c

@@ -582,6 +582,7 @@ clear_level_structures()
 	level.flags.is_cavernous_lev = 0;
 	level.flags.arboreal = 0;
 	level.flags.wizard_bones = 0;
+        level.flags.corrmaze = 0;
 
 	nroom = 0;
 	rooms[0].hx = -1;