extensions: libxt_conntrack: Add translation to nft

extensions: libxt_conntrack: Add translation to nft

Add translation of conntrack to nftables.

Examples:

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctstate NEW,RELATED -j ACCEPT
nft add rule ip filter INPUT ct state new,related counter accept

$ sudo ip6tables-translate -t filter -A INPUT -m conntrack ! --ctstate NEW,RELATED -j ACCEPT
nft add rule ip6 filter INPUT ct state != new,related counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctproto UDP -j ACCEPT
nft add rule ip filter INPUT ct proto 17 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack ! --ctproto UDP -j ACCEPT
nft add rule ip filter INPUT ct proto != 17 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctorigsrc 10.100.2.131 -j ACCEPT
nft add rule ip filter INPUT ct original saddr 10.100.2.131 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctorigsrc 10.100.0.0/255.255.0.0 -j ACCEPT
nft add rule ip filter INPUT ct original saddr 10.100.0.0/16 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctorigdst 10.100.2.131 -j ACCEPT
nft add rule ip filter INPUT ct original daddr 10.100.2.131 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctreplsrc 10.100.2.131 -j ACCEPT
nft add rule ip filter INPUT ct reply saddr 10.100.2.131 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctrepldst 10.100.2.131 -j ACCEPT
nft add rule ip filter INPUT ct reply daddr 10.100.2.131 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctproto tcp --ctorigsrcport 443:444 -j ACCEPT
nft add rule ip filter INPUT ct original protocol 6 ct original proto-src 443-444 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack ! --ctstatus CONFIRMED -j ACCEPT
nft add rule ip filter INPUT ct status != confirmed counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctexpire 3 -j ACCEPT
nft add rule ip filter INPUT ct expiration 3 counter accept

$ sudo iptables-translate -t filter -A INPUT -m conntrack --ctdir ORIGINAL -j ACCEPT
nft add rule ip filter INPUT ct direction original counter accept

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