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/mkobj.c

@@ -2130,6 +2130,8 @@ struct obj *obj;
 
     obj->where = OBJ_MIGRATING;
     obj->nobj = g.migrating_objs;
+    obj->omigr_from_dnum = u.uz.dnum;
+    obj->omigr_from_dlevel = u.uz.dlevel;
     g.migrating_objs = obj;
 }