Convert `Unix{Datagram,Stream}::{set_}passcred()` to per-OS traits

These methods are the pre-stabilized API for obtaining peer credentials
from an `AF_UNIX` socket, part of the `unix_socket_ancillary_data` feature.

Their current behavior is to get/set one of the `SO_PASSCRED` (Linux),
`LOCAL_CREDS_PERSISTENT` (FreeBSD), or `LOCAL_CREDS` (NetBSD) socket
options. On other targets the `{set_}passcred()` methods do not exist.

There are two problems with this approach:

1. Having public methods only exist for certain targets isn't permitted
   in a stable `std` API.

2. These options have generally similar purposes, but they are non-POSIX
   and their details can differ in subtle and surprising ways (such as
   whether they continue to be set after the next call to `recvmsg()`).

Splitting into OS-specific extension traits is the preferred solution to
both problems.

1 files changed, 3 insertions, 0 deletions

diff --git a/library/std/src/os/linux/net.rs b/library/std/src/os/linux/net.rs
index c8e734d740b..f898e705487 100644
--- a/library/std/src/os/linux/net.rs
+++ b/library/std/src/os/linux/net.rs
@@ -5,5 +5,8 @@
 #[stable(feature = "unix_socket_abstract", since = "1.70.0")]
 pub use crate::os::net::linux_ext::addr::SocketAddrExt;
 
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+pub use crate::os::net::linux_ext::socket::UnixSocketExt;
+
 #[unstable(feature = "tcp_quickack", issue = "96256")]
 pub use crate::os::net::linux_ext::tcp::TcpStreamExt;