extensions: libxt_cgroup: Add translation to nft

extensions: libxt_cgroup: Add translation to nft

Add translation for cgroup to nft. Path parameter not supported in nft
yet.

Examples:

$ sudo iptables-translate -t filter -A INPUT -m cgroup --cgroup 0 -j ACCEPT
nft add rule ip filter INPUT meta cgroup 0 counter accept

$ sudo iptables-translate -t filter -A INPUT -m cgroup ! --cgroup 0 -j ACCEPT
nft add rule ip filter INPUT meta cgroup != 0 counter accept

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>