unix: add PID to Xucred on FreeBSD

[rwinkhart]

This PR adds support for getting the PID of a process from FreeBSD's Xucred.
I find this to be essential functionality for RPC binary authentication
(using the PID to find the path of a binary to authenticate based on
the hash of said binary).

I'm going to do something unconventional and list some reasons this
probably cannot be merged:

1. This only works on FreeBSD 13+
2. I made manual edits to files that state I should not.
   I admit I probably should have learned the tooling of this project
   so that I could correctly generate these files, but the way these
   files are generated is admittedly a bit over my head and I could not
   quite figure it out. This probably needs to be addressed.

Despite these, I figured I'd submit this and see what comes of it. Thank you!

Updates golang/go#41659

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Patch Set 1:

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