[libc-0.2] Freebsd Netlink support backported

[ydirson]

This is a proposal to bring #3201 and #3367 to libc-0.2 branch, by hiding this change behind a feature flag as suggested by @asomers in https://github.com/rust-lang/libc/pull/3367#issuecomment-1860931763.

This makes it possible to build on FreeBSD 13 eg. sysinfo v0.30.5 from @GuillaumeGomez it the feature flag is not specified, and to build rtnetlink from PR https://github.com/rust-netlink/rtnetlink/pull/49 if it is specified.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[bors]

:umbrella: The latest upstream changes (presumably #3589) made this pull request unmergeable. Please resolve the merge conflicts.

[ydirson]

Rebased.

Is there anything I can do to make this PR go forward?

[ydirson]

Rebased to allow replacing current version with cargo patch: 0.2.155