make fname_rules address family dependent
IF_NETMASK is not actually used
add rules for preferred prefixes at a lower priority than the rules binding interface addresses to a particular routing table
IPv6 support

unfortunately we can't just do the exact same thing that we do for IPv4
except with "ip -6", but we can do the next best thing.  this doesn't
support multi_gateway_weight, but it does support multi_preferred_prefixes.
however, except for the preferred prefixes, outbound connections will
only use the first configured IPv6 default gateway.