Support for `getpeereid(3)`

[woodruffw]

macOS and most of the BSDs have a getpeereid(3) library routine, which can be used to retrieve the effective UID and GID for a given Unix domain socket.

This roughly mirrors the functionality provided by getsockopt(2) + SO_PEERCRED on Linux (minus the PID), which the nix crate already supports.

I have the bandwidth and ability to implement this, if desired.

[valpackett]

We'll need this in zbus, so you have a consumer already!

---

Side note, OpenBSD also supports SO_PEERCRED with minor differences (usage example); FreeBSD supports LOCAL_PEERCRED but gained the pid field very recently; NetBSD has LOCAL_PEEREID; Solaris/illumos(?) has getpeerucred. Would be nice to also get all these mechanisms in here eventually…

[woodruffw]

Fantastic. I have rust-lang/rust#79387 pending on Rust itself to improve the nightly-only UCred API, which should eventually handle those minor differences on the BSDs as well. But yeah, it would certainly good to have them exposed independently in nix as well.

[woodruffw]

Okay, #1342 has the wrapper.