Make regression/priv compile again after the multi-IP jail
changes. Note that we are still using the legacy jail(2)
rather than the jail_set(2)/jail(3) syscall.
Add an IPv4, and an IPv6 loopback address in case we compile
with INET6 enabled.
Make the priv_vfs_extattr_system compile on amd64 as well using the
proper length modifier to printf(3) for ssize_t.
Reviewed by: rwatson
Approved by: re (kib)