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/you.h

@@ -1,4 +1,4 @@
-/* NetHack 3.7	you.h	$NHDT-Date: 1596498576 2020/08/03 23:49:36 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.48 $ */
+/* NetHack 3.7	you.h	$NHDT-Date: 1686726254 2023/06/14 07:04:14 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.72 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Robert Patrick Rankin, 2016. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -471,6 +471,8 @@ struct you {
     int uinvault;
     struct monst *ustuck;    /* engulfer or grabber, maybe grabbee if Upolyd */
     struct monst *usteed;    /* mount when riding */
+    unsigned ustuck_mid;     /* u.ustuck->m_id, used during save/restore */
+    unsigned usteed_mid;     /* u.usteed->m_id, used during save/restore */
     long ugallop;            /* turns steed will run after being kicked */
     int urideturns;          /* time spent riding, for skill advancement */
     int umortality;          /* how many times you died */