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.

doc/fixes34.1

@@ -315,6 +315,7 @@ only hard helmets protect against falling piercers
 don't crash teleporting out of a monster while engulfed, punished but not
 	carrying the ball
 web breaking should consider steed strength and other characteristics
+various missing or inappropriate "killed by" death messages
 
 
 Platform- and/or Interface-Specific Fixes

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;

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)) {

src/music.c

@@ -409,7 +409,7 @@ struct obj *instr;
 		    if ((damage = zapyourself(instr, TRUE)) != 0) {
 			char buf[BUFSZ];
 			Sprintf(buf, "using a magical horn on %sself", uhim());
-			losehp(damage, buf, NO_KILLER_PREFIX);
+			losehp(damage, buf, KILLED_BY);
 		    }
 		} else {
 		    buzz((instr->otyp == FROST_HORN) ? AD_COLD-1 : AD_FIRE-1,

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 =
+			killer = "deliberately meeting Medusa's gaze";
-			 "deliberately gazing at Medusa's hideous countenance";
 			done(STONING);
 		    }
 		}