netfilter: connlimit: split xt_connlimit into front and backend

This allows to reuse xt_connlimit infrastructure from nf_tables.
The upcoming nf_tables frontend can just pass in an nftables register
as input key, this allows limiting by any nft-supported key, including
concatenations.

For xt_connlimit, pass in the zone and the ip/ipv6 address.

With help from Yi-Hung Wei.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>