ifa_preferred
ifa_preferred(ifa_maybe, ifa) ||
int ifa_preferred(struct ifaddr *, struct ifaddr *);
(best == NULL || ifa_preferred(best, ifa)))
if (ifa_preferred(&ia_best->ia_ifa, &ia->ia_ifa))
if (ifa_preferred(&ia->ia_ifa, &ia_best->ia_ifa))