Fix the definition of sigevent on FreeBSD and Linux

[asomers]

It was originally defined back before rust could represent C unions. So instead of defining the union field correctly, it simply defined that union's most useful field. Define it correctly now.

Remove traits that can't be safely implemented on a union: PartialEq, Eq, and Hash. Define Debug, but exclude the union field. Leave Clone in place.

This PR is a rebase of #2813 that eschews all of the backwards-compatibility parts.

[rustbot]

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[JohnTitor]

Failed on musl: https://github.com/rust-lang/libc/actions/runs/8846424497/job/24292411184?pr=3630

[asomers]

@JohnTitor I fixed the build failure on musl and rebased on main

[asomers]

The Sparc failure is not due to this PR.

[SteveLauC]

Considering the main branch is for version 1.0, which allows breaking changes, I think we can move this forward.