<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.
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