support for SO_PEERCRED

[cgwalters]

In one project I'm using SO_PEERCRED: https://github.com/coreos/bootupd/blob/main/src/ipc.rs - looking at using this crate instead. What do you think about exposing that via an API?

The usual pattern here is to pass credentials on the first message. In the above code I represented this as a state machine encoded via structs; e.g. perhaps we'd add RawAuthenticatingSender and RawUnauthenticatedReceiver, where e.g.

impl RawUnauthenticatedReceiver {
   pub async fn recv(self) -> Result<(RawReceiver, Credentials, Vec<u8>, Option<Vec<RawFd>>)>

Note this consumes the unauthenticated type and returns credentials. Would need to debate exposing e.g. https://docs.rs/nix/latest/nix/sys/socket/struct.UnixCredentials.html or (probably better for semver) a new struct here.

Alternatively we could add recv_with_creds() and send_with_creds() to the raw types.

[mitsuhiko]

Good question. While I think the RawUnauthenticatedReceiver is interesting I wonder if it's not in practice too impractical to have to swap the type after the first message. I'm slightly favoring the recv_with_creds option. This would also translate better through the serde version if needed.