Tracking Issue for socket `set_mark`

[devnexen]

This is a tracking issue for the socket so_mark new function for this PR

Public API

// core::magic

fn main() -> std::io::Result<()> {
   let sockid: u32 = 32;
   let socket = UnixDatagram::unbound()?;
   socket.set_mark(sockid)
   Ok(())
}

• Implementation: #...
• Final comment period (FCP)
• Stabilization PR