fix up several death messages

<Someone> reported several incorrect death messages
1) "petrified by deliberately gazing at Medusa's hideous countenance" is
too long and won't fit on the tombstone.  I reworded it, which also better
reflects that Medusa's gaze is really an active attack.
2) "killed by war hammer named Mjollnir" for partly identified Mjollnir now
says "killed by a war hammer named Mjollnir".
3) "using a magical horn on himself" was missing the "killed by" prefix
4) there were supposedly cases the the a/an article was missing after being
killed by a monster.  I didn't see where this was occuring (eg AoLS resets
killer_format before it returns), but now done_in_by always resets
killer_format, which should address any such cases.

src/end.c

@@ -190,6 +190,7 @@ register struct monst *mtmp;
 	You("die...");
 	mark_synch();	/* flush buffered screen output */
 	buf[0] = '\0';
+	killer_format = KILLED_BY_AN;
 	/* "killed by the high priest of Crom" is okay, "killed by the high
 	   priest" alone isn't */
 	if ((mtmp->data->geno & G_UNIQ) != 0 && !(mtmp->data == &mons[PM_HIGH_PRIEST] && !mtmp->ispriest)) {