Add Rust type to represent a valid libc::sockaddr_ll

[jonatanzeidler]

What does this PR do

This PR adds a SockAddrLl struct to represent a libc::socketaddr_ll with the additional guarantee of valid data, that can be made into a LinkAddr without unsafe code. See #2327, #2059 and #2396 for reference.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API

[jonatanzeidler]

Just for clarification: Is the unsafe around constructing the LinkAddr due to the invariant (like len must match the other data) or about the potentially unsound libc types coming in via an FFI?

[asomers]

Just for clarification: Is the unsafe around constructing the LinkAddr due to the invariant (like len must match the other data) or about the potentially unsound libc types coming in via an FFI?

If you're referring to LinkAddr::from_raw, it must be unsafe because it reads from a raw pointer. All methods that read from a raw pointer are unsafe. And the reason that it reads from a raw pointer is because that's how many C APIs, like getpeeraddr provide sockaddrs. C basically treats the various sockaddr types as a variable length tagged union.

This PR's implementation of From<SockAddrLl> is incorrect. It doesn't guarantee valid data. It doesn't validate the family or length fields, so it's easy to construct an invalid sockaddr_ll. Is all you want a safe constructor for sockaddr_ll? If so, I suggest you simply add one. There's no need for a new type.