Use Rust union types

[kellda]

For #1020

Replace some types with unions now that rustc supports them.

I also found sigevent.sigev_notify_thread_id, siginfo_t.__data and sigaction.sa_sigaction that I think should be unions, but I wasn't sure.

[rust-highfive]

r? @Amanieu

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[kellda]

Also I had to ignore iffaddrs.ifa_ifu on Linux to pass CI

[Amanieu]

Unfortunately I believe this is a breaking change, which means we can't actually do this until we bump the crate version to 1.0.

cc @JohnTitor

[kellda]

I'm aware that it is a breaking change and that's not a problem for me to wait until a new major version for this to be merged.

[bors]

☔ The latest upstream changes (presumably #1975) made this pull request unmergeable. Please resolve the merge conflicts.