fix github issue #531 - genderless corpses

fix github issue #531 - genderless corpses

Dead monsters that had traits saved with the corpse would revive as
the same gender, but ordinary corpses revived with random gender so
could be different from before they got killed.

Since corpses of monsters lacked gender, those for monsters with
gender-specific names were described by the neuter name.

This is a fairly big change for a fairly minor problem and needs a
lot more testing.

Fixes #531