Fix #812: recovered stair dlevel

Stair dlevels weren't being restored with the correct values when
recovered after the game crashed, apparently because they weren't being
reset back to their 'absolute' level from a 'relative' level.  I'm not
totally sure of why this affected only recovered games (maybe that's the
only time when the 'relative' stair values are used?) but this fix seems
to work.

Fixes #812

include/hack.h

@@ -254,6 +254,11 @@ enum saveformats {
     ascii = 3           /* each field, ascii text (just proof of concept) */
 };
 
+enum restore_stages {
+    REST_GSTATE = 1,    /* restoring current level and game state */
+    REST_LEVELS = 2,    /* restoring remainder of dungeon */
+};
+
 /* sortloot() return type; needed before extern.h */
 struct sortloot_item {
     struct obj *obj;