buglist - full level triggers impossible() from migrating mons

<email deleted> wrote:
> If more monsters fall through a trap door than can fit on the
> level below, when you go down the stairs, you get the following
> message:
>  "Program in disorder - perhaps you'd better #quit.
>  rloc(): couldn't relocate monster"
> This message seems to appear once for every monster-too-many that
> fell through the hole. I originally found this while
> intentionally completely filling a level with black puddings
> (there was a trap door I didn't know about). I also confirmed it
> in a wiz-mode test using gremlins and water.

[confirmed: moveloop -> deferred_goto -> goto_level ->
 losedogs -> mon_arrive -> rloc -> impossible]

This patch:
- causes rloc() to return TRUE if successful,
  or FALSE if it wasn't.
- adds code to mon_arrive() in dog.c to deal with
  the failed rloc()
- allows the x,y parameters to mkcorpstat() to
  be 0,0 in order to trigger random placement of the
  corpse on the level
- if you define DEBUG_MIGRATING_MONS when you build cmd.c
  then you'll have a debug-mode command #migratemons to
  store the number of random monsters that you specify
  on the migrating monsters chain.

src/mhitm.c

@@ -839,7 +839,7 @@ mdamagem(magr, mdef, mattk)
 		       we'll get "it" in the suddenly disappears message */
 		    if (vis) Strcpy(mdef_Monnam, Monnam(mdef));
 		    mdef->mstrategy &= ~STRAT_WAITFORU;
-		    rloc(mdef);
+		    (void) rloc(mdef, FALSE);
 		    if (vis && !canspotmon(mdef)
 #ifdef STEED
 		    	&& mdef != u.usteed
@@ -967,7 +967,7 @@ mdamagem(magr, mdef, mattk)
 		    pline("%s steals some gold from %s.", buf, mon_nam(mdef));
 		}
 		if (!tele_restrict(magr)) {
-		    rloc(magr);
+		    (void) rloc(magr, FALSE);
 		    if (vis && !canspotmon(magr))
 			pline("%s suddenly disappears!", buf);
 		}
@@ -1034,7 +1034,7 @@ mdamagem(magr, mdef, mattk)
 							0 : MM_AGR_DIED));
 			if (magr->data->mlet == S_NYMPH &&
 			    !tele_restrict(magr)) {
-			    rloc(magr);
+			    (void) rloc(magr, FALSE);
 			    if (vis && !canspotmon(magr))
 				pline("%s suddenly disappears!", buf);
 			}