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.
