Track SCM_RIGHTS support via socket2 or std

[seuros]

Context

The tcp_listener_fd_passing example (PR #778) requires libc for SCM_RIGHTS file descriptor passing because:

• rama::unix provides UnixListener/UnixStream for socket creation and cleanup, but no ancillary data (SCM_RIGHTS) support
• std::os::unix::net::SocketAncillary is still nightly-only (#![feature(unix_socket_ancillary_data)])

Goal

Remove libc dependency once socket2 or std provides stable SCM_RIGHTS support, then add safe wrappers to rama::unix.

Upstream tracking

• socket2: Support for ancillary data/control messages in sendmsg/recvmsg as well as the upcoming sendmmsg/recvmmsg rust-lang/socket2#614

[GlenDC]

Thank you for tracking this @seuros ; for now it will remain here. Worst case I might add some of our own unsafe bindings in next month or so. For now it can remain open, unless someone needs this sooner. Most likely we can wait on upstream fixing it.