Rework stairs structure

Use a linked list to store stair and ladder information, instead
of having fixed up/down stairs/ladders and a single "special" (branch)
stair.

Breaks saves and bones.

Adds information to migrating objects and monsters for the dungeon
and level where they are migrating from.

src/zap.c

@@ -2891,6 +2891,7 @@ struct obj *obj; /* wand or spell */
     struct engr *e;
     struct trap *ttmp;
     char buf[BUFSZ];
+    stairway *stway = g.stairs;
 
     /* some wands have special effects other than normal bhitpile */
     /* drawbridge might change <u.ux,u.uy> */
@@ -2913,11 +2914,16 @@ struct obj *obj; /* wand or spell */
         return TRUE; /* we've done our own bhitpile */
     case WAN_OPENING:
     case SPE_KNOCK:
+        while (stway) {
+            if (!stway->isladder && !stway->up && stway->tolev.dnum == u.uz.dnum)
+                break;
+            stway = stway->next;
+        }
         /* up or down, but at closed portcullis only */
         if (is_db_wall(x, y) && find_drawbridge(&xx, &yy)) {
             open_drawbridge(xx, yy);
             disclose = TRUE;
-        } else if (u.dz > 0 && (x == xdnstair && y == ydnstair)
+        } else if (u.dz > 0 && stway && stway->sx == x && stway->sy == y
                    /* can't use the stairs down to quest level 2 until
                       leader "unlocks" them; give feedback if you try */
                    && on_level(&u.uz, &qstart_level) && !ok_to_quest()) {