socket: support TCP_CORK option in Linux

[cgzones]

Closes: #2758

What does this PR do

Enable getting and setting the corking state of a tcp connection. See man:tcp(7).

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

[kamalmarhubi]

Thanks @cgzones! Can you also add a changelog/2769.added.md file for this? 🙏

[SteveLauC]

I rebased your branch to resolve the CI failures.

[SteveLauC]

It would be great if we could have a simple round-trip test for this binding:

#[cfg(target_os = "linux")]
#[test]
fn test_tcp_cork() {
    let fd = socket(
        AddressFamily::Inet,
        SockType::Stream,
        SockFlag::empty(),
        SockProtocol::Tcp,
    )
    .unwrap();

    setsockopt(&fd, sockopt::TcpCork, &true).unwrap();
    assert!(getsockopt(&fd, sockopt::TcpCork).unwrap());

    setsockopt(&fd, sockopt::TcpCork, &false).unwrap();
    assert!(!getsockopt(&fd, sockopt::TcpCork).unwrap());
}

[cgzones]

It would be great if we could have a simple round-trip test for this binding:

added

[SteveLauC]

Thanks