netfilter: nf_tables: Add synproxy support

Add synproxy support for nf_tables. This behaves like the iptables
synproxy target but it is structured in a way that allows us to propose
improvements in the future.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>