x/sys/unix: want more platform support for UNIX domain socket peer credentials

What version of Go are you using (go version)?

(Although I use many different versions of Go on illumos, Linux, darwin, etc.)

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Feature request

As this is a feature request, I'm not including typical bug report details.

What I'd like is a way for us to obtain peer credentials more broadly. I am most interested in illumos/solaris (which uses getpeerucred() in libc, but I see this being useful for Darwin, FreeBSD, and possibly others.

With Linux we can do GetsockoptUcred with SO_PEERCRED. Which is great. I would like to have a similar way to do this for other platforms (platform specific probably) without having to resort to cgo if possible.