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.