Auto merge of #117156 - jmillikin:os-unix-socket-ext, r=Amanieu,dtolnay

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.