extensions: add SYNPROXY extension

Signed-off-by: Patrick McHardy <kaber@trash.net>
utils: add nfsynproxy tool

[ Originally synconf, but Jesper D. Brouer suggested to change
  the name to avoid a possible filename clash. I also include
  nfsynproxy in the final configure report --pablo ]

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
extensions: libxt_set, libxt_SET: check the set family too

Do not accept silently sets with wrong protocol family but reject
them with an error message. It makes straightforward to catch user
errors.

[ Use afinfo instead to avoid a binary interface update --pablo ]

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>