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.

include/obj.h

@@ -122,6 +122,8 @@ struct obj {
     long age;               /* creation date */
     long owornmask;
     unsigned lua_ref_cnt;  /* # of lua script references for this object */
+    xchar omigr_from_dnum; /* where obj is migrating from */
+    xchar omigr_from_dlevel; /* where obj is migrating from */
     struct oextra *oextra; /* pointer to oextra struct */
 };