Make netfilter constants available for musl Linux targets

[pulsastrix]

The netfilter constants added in #911 and #926 are currently only available for Linux targets using glibc because they weren't available in the musl-sanitized kernel headers at the time these PRs were made.

With current versions of the sanitized headers, these constants are available, and this PR moves them to unix/linux_like/linux/mod.rs so that they can be used on targets using musl libc.

The kernel header version currently set in ci/install_musl.sh already supports these constants, but has different values for e.g. NFT_TABLE_MAXNAMELEN than the ones that were already defined for glibc. It seems like the maximum name length for various netfilter objects has been changed in the kernel (the respective commits are 1 2 3 and 4). To match these values with the ones that were already defined, this PR also updates the used kernel header version in ci/install_musl.sh.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[JohnTitor]

Any reason to mark this PR as WIP?

[pulsastrix]

Not really, I'll mark it as ready.

[JohnTitor]

Thanks!
@bors r+

[bors]

📌 Commit bf8378b has been approved by JohnTitor

[bors]

⌛ Testing commit bf8378b with merge 802f327...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 802f327 to master...