For FreeBSD 12.2 and before I think we could use SCM_CREDS to get
the pid of the peer. For FreeBSD 13.x and later, the Xucred is
extended with a pid. For now we'll leave it unimplemented and
expect to add FreeBSD 13+ support whern it is released.
Signed-off-by: Denton Gentry <dgentry@tailscale.com>