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.

include/config.h

@@ -416,7 +416,6 @@ typedef unsigned char	uchar;
 /* display features */
 /* dungeon features */
 /* dungeon levels */
-#define WALLIFIED_MAZE	/* Fancy mazes - Jean-Christophe Collet */
 /* monsters & objects */
 /* I/O */
 #if !defined(MAC)