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/mon.c

@@ -2327,11 +2327,15 @@ register struct monst *mtmp;
 #endif
 
     if (mtmp->data->mlet == S_KOP) {
+        stairway *stway = stairway_find_type_dir(FALSE, FALSE);
+
         /* Dead Kops may come back. */
         switch (rnd(5)) {
         case 1: /* returns near the stairs */
-            (void) makemon(mtmp->data, xdnstair, ydnstair, NO_MM_FLAGS);
-            break;
+            if (stway) {
+                (void) makemon(mtmp->data, stway->sx, stway->sy, NO_MM_FLAGS);
+                break;
+            }
         case 2: /* randomly */
             (void) makemon(mtmp->data, 0, 0, NO_MM_FLAGS);
             break;