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/dothrow.c

@@ -1006,7 +1006,8 @@ boolean twoweap; /* used to restore twoweapon mode if wielded weapon returns */
 				  body_part(ARM));
 			    (void) artifact_hit((struct monst *)0,
 						&youmonst, obj, &dmg, 0);
-			    losehp(dmg, xname(obj), KILLED_BY);
+			    losehp(dmg, xname(obj),
+				obj_is_pname(obj) ? KILLED_BY : KILLED_BY_AN);
 			}
 			if (ship_object(obj, u.ux, u.uy, FALSE))
 			    return;