mon_leaving_level

From 6 year old email:  m_detach (monster death or removal from play)
and relmon (monster migrating to another level) both take a monster
off the map but they weren't consistent with each other.  Change them
to use a common routine for that.

I'm not sure whether the inconsistencies resulted in any bugs.  The
email was concerned about handling for monsters that emit light, but
those aren't actually common to the two removal methods and turned
out to be ok.

src/dog.c

@@ -325,6 +325,7 @@ mon_arrive(struct monst *mtmp, boolean with_you)
     /* some monsters might need to do something special upon arrival
        _after_ the current level has been fully set up; see dochug() */
     mtmp->mstrategy |= STRAT_ARRIVE;
+    mtmp->mstate &= ~MON_MIGRATING;
 
     /* make sure mnexto(rloc_to(set_apparxy())) doesn't use stale data */
     mtmp->mux = u.ux, mtmp->muy = u.uy;
@@ -480,8 +481,9 @@ mon_arrive(struct monst *mtmp, boolean with_you)
 
 /* heal monster for time spent elsewhere */
 void
-mon_catchup_elapsed_time(struct monst *mtmp,
-                         long nmv) /* number of moves */
+mon_catchup_elapsed_time(
+    struct monst *mtmp,
+    long nmv) /* number of moves */
 {
     int imv = 0; /* avoid zillions of casts and lint warnings */
 
@@ -689,10 +691,11 @@ keepdogs(boolean pets_only) /* true for ascension or final escape */
 }
 
 void
-migrate_to_level(struct monst *mtmp,
-                 xchar tolev, /* destination level */
-                 xchar xyloc, /* MIGR_xxx destination xy location: */
-                 coord *cc)   /* optional destination coordinates */
+migrate_to_level(
+    struct monst *mtmp,
+    xchar tolev, /* destination level */
+    xchar xyloc, /* MIGR_xxx destination xy location: */
+    coord *cc)   /* optional destination coordinates */
 {
     struct obj *obj;
     d_level new_lev;
@@ -724,6 +727,7 @@ migrate_to_level(struct monst *mtmp,
         m_unleash(mtmp, TRUE);
     }
     relmon(mtmp, &g.migrating_mons); /* move it from map to g.migrating_mons */
+    mtmp->mstate |= MON_MIGRATING;
 
     new_lev.dnum = ledger_to_dnum((xchar) tolev);
     new_lev.dlevel = ledger_to_dlev((xchar) tolev);
@@ -743,6 +747,11 @@ migrate_to_level(struct monst *mtmp,
     mtmp->mux = new_lev.dnum;
     mtmp->muy = new_lev.dlevel;
     mtmp->mx = mtmp->my = 0; /* this implies migration */
+
+    /* don't extinguish a mobile light; it still exists but has changed
+       from local (monst->mx > 0) to global (mx==0, not on this level) */
+    if (emits_light(mtmp->data))
+        vision_recalc(0);
 }
 
 /* return quality of food; the lower the better */