Auto merge of #115159 - solid-rs:patch/kmc-solid/io-safety, r=workingjubilee

kmc-solid: I/O safety

Adds the I/O safety API (#87329) for socket file descriptors in [`*-kmc-solid_*`](https://doc.rust-lang.org/nightly/rustc/platform-support/kmc-solid.html) Tier 3 targets. All new public items are gated by the `solid_ext` library feature.

This PR adds the following public types and traits:

    std::os::solid::io::AsFd
    std::os::solid::io::BorrowedFd
    std::os::solid::io::OwnedFd

    std::os::solid::prelude::AsFd (re-export)
    std::os::solid::prelude::BorrowedFd (re-export)
    std::os::solid::prelude::OwnedFd (re-export)

And trait implementations:

    From<std::net::TcpListener> for std::os::solid::io::OwnedFd
    From<std::net::TcpStream> for std::os::solid::io::OwnedFd
    From<std::net::UdpSocket> for std::os::solid::io::OwnedFd
    From<std::os::solid::io::OwnedFd> for std::net::TcpListener
    From<std::os::solid::io::OwnedFd> for std::net::TcpStream
    From<std::os::solid::io::OwnedFd> for std::net::UdpSocket
    std::fmt::Debug for std::os::solid::io::BorrowedFd<'_>
    std::fmt::Debug for std::os::solid::io::OwnedFd
    std::io::IsTerminal for std::os::solid::io::BorrowedFd<'_>
    std::io::IsTerminal for std::os::solid::io::OwnedFd
    std::os::fd::AsRawFd for std::os::solid::io::BorrowedFd<'_>
    std::os::fd::AsRawFd for std::os::solid::io::OwnedFd
    std::os::fd::FromRawFd for std::os::solid::io::OwnedFd
    std::os::fd::IntoRawFd for std::os::solid::io::OwnedFd
    std::os::solid::io::AsFd for &impl std::os::solid::io::AsFd
    std::os::solid::io::AsFd for &mut impl std::os::solid::io::AsFd
    std::os::solid::io::AsFd for Arc<impl std::os::solid::io::AsFd>
    std::os::solid::io::AsFd for Box<impl std::os::solid::io::AsFd>
    std::os::solid::io::AsFd for Rc<impl std::os::solid::io::AsFd>
    std::os::solid::io::AsFd for std::net::TcpListener
    std::os::solid::io::AsFd for std::net::TcpStream
    std::os::solid::io::AsFd for std::net::UdpSocket
    std::os::solid::io::AsFd for std::os::solid::io::BorrowedFd<'_>
    std::os::solid::io::AsFd for std::os::solid::io::OwnedFd

Taking advantage of the above change, this PR also refactors the internal details of `std::sys::solid::net` to match the design of other targets, e.g., by redefining `Socket` as a newtype of `OwnedFd`.

Author: bors