fix #3841 - steed and engulfer on same map spot

Reported five months ago, a save was performed while a mounted hero
was engulfed.  Restore issued a warning about the engulfer being
placed on top of the steed (who shouldn't have been on the map).

The report arrived at about the same time as engulfing a riding
hero was changed to force a dismount instead of engulfing both hero
and steed so nothing further was done about it.  This changes
restore to not put a steed on the map and then take it off again.
It also attempts to simplify usteed and ustuck handling during save
and restore.

Testing so far indicates that things are still working correctly.
Keep makeplural(body_part(FINGER)) crossed.

Existing save and bones files are invalidated.

include/decl.h

@@ -1,4 +1,4 @@
-/* NetHack 3.7  decl.h  $NHDT-Date: 1657918080 2022/07/15 20:48:00 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.303 $ */
+/* NetHack 3.7  decl.h  $NHDT-Date: 1686726249 2023/06/14 07:04:09 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.333 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Michael Allison, 2007. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -968,8 +968,6 @@ struct instance_globals_u {
     struct Race urace; /* player's race. May be munged in role_init() */
 
     /* save.c */
-    unsigned ustuck_id; /* need to preserve during save */
-    unsigned usteed_id; /* need to preserve during save */
     d_level uz_save;
 
     /* new stuff */