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/rm.h

@@ -554,6 +554,8 @@ struct levelflags {
 	Bitfield(wizard_bones,1);       /* set if level came from a bones file
 					   which was created in wizard mode (or
 					   normal mode descendant of such) */
+        Bitfield(corrmaze, 1);          /* Whether corridors are used for the maze
+                                           rather than ROOM */
 };
 
 typedef struct