netfilter: connlabels: move helpers to xt_connlabel

Currently labels can only be set either by iptables connlabel
match or via ctnetlink.

Before adding nftables set support, clean up the clabel core and move
helpers that nft will not need after all to the xtables module.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>