Akin to nix::sys::socket::SockFlag but avail cross-platform
Like accept4, falls back to non-atomic accept
Intended to check for completion after
Falls back to non-atomic if SOCK_NONBLOCK/SOCK_CLOEXEC unavailable
Count of bytes that have yet to be read from a socket
Count of bytes that have been written to a socket, but have yet to be acked by the remote end. Works on Android, Linux, macOS, iOS, FreeBSD and NetBSD, returns 0 on others.