Use the IfaCacheinfo type and the IFA_* consts from golang.org/x/sys/unix instead of locally duplicating them.
This adds parsing of the preferred and valid lifetime information from the netlink IFA_CACHEINFO attribute. They are stored as PreferedLft and ValidLft in the Addr struct if found. Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>