Monsters shouldn't use wands of digging in Sokoban

A number of players have complained that Sokoban can be rendered
unsolvable without "creative nethacking" when monsters zap wands of
digging.  This patch prevents monsters from selecting wands of
digging in Sokoban.

Note that we can't simply make Sokoban HARDFLOOR, as that causes
problems with the hole generation code.

doc/fixes33.2

@@ -394,6 +394,7 @@ golden glow when praying will recover lost level if blessed full healing could
 gaining a level while polymorphed increases current monst hit points as well
 	as latent human (or whatever) hit points
 pets should not try to go after food that they can't reach
+monsters shouldn't use wands of digging in Sokoban
 
 
 Platform- and/or Interface-Specific Fixes

src/muse.c

@@ -433,6 +433,8 @@ struct monst *mtmp;
 		if (obj->otyp == WAN_DIGGING && obj->spe > 0 && !stuck && !t
 		    && !mtmp->isshk && !mtmp->isgd && !mtmp->ispriest
 		    && !is_floater(mtmp->data)
+		    /* monsters digging in Sokoban can ruin things */
+		    && !In_sokoban(&u.uz)
 		    /* digging wouldn't be effective; assume they know that */
 		    && !(levl[x][y].wall_info & W_NONDIGGABLE)
 		    && !(Is_botlevel(&u.uz) || In_endgame(&u.uz))