libxtables: find extensions based on family too

libxtables: find extensions based on family too

When using libxtables with an external program (nft) which switches family
contexts (using xtables_set_nfproto()), the xtables_find_{match,target}
functions need to compare the family too.

We want to avoid this situation:

 1) user first sets afinfo to IPv6
 2) xtables_find_target() finds & load ip6t_REJECT and uses it
 3) afinfo change to IPv4
 4) user then tries to use ipt_REJECT
 5) xtables_find_target() finds ip6t_REJECT instead (same target name)
 6) using ip6t_REJECT as ipt_REJECT can cause a lot of troubles

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>