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/polyself.c

@@ -1020,8 +1020,7 @@ dogaze()
 			/* as if gazing at a sleeping anything is fruitful... */
 			You("turn to stone...");
 			killer_format = KILLED_BY;
-			killer =
-			 "deliberately gazing at Medusa's hideous countenance";
+			killer = "deliberately meeting Medusa's gaze";
 			done(STONING);
 		    }
 		}