Support reading ucred from the socket on FreeBSD

On FreeBSD we have to use getsockopt(fd, SOL_LOCAL, LOCAL_PEERCRED)
instead. This change is based on a downstream patch in FreeBSD ports.

Co-authored-by: Greg V <greg@unrelenting.technology>
Co-authored-by: Koop Mast <kwm@rainbow-runner.nl>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>