extensions: libip6t_DNAT: Add translation to nft

extensions: libip6t_DNAT: Add translation to nft

Add translation for target DNAT to nftables.

Examples:

$ sudo ip6tables-translate -t nat -A prerouting -i eth1 -p tcp --dport 8080 -j DNAT --to-destination [fec0::1234]:80
nft add rule ip6 nat prerouting iifname eth1 tcp dport 8080 counter dnat fec0::1234 :80

$ sudo ip6tables-translate -t nat -A prerouting -p tcp -j DNAT --to-destination [fec0::1234]:1-20
nft add rule ip6 nat prerouting ip6 nexthdr tcp counter dnat fec0::1234 :1-20

$ sudo ip6tables-translate -t nat -A prerouting -p tcp -j DNAT --to-destination [fec0::1234]:80 --persistent
nft add rule ip6 nat prerouting ip6 nexthdr tcp counter dnat fec0::1234 :80 persistent

$ sudo ip6tables-translate -t nat -A prerouting -p tcp -j DNAT --to-destination [fec0::1234]:80 --random --persistent
nft add rule ip6 nat prerouting ip6 nexthdr tcp counter dnat fec0::1234 :80 random,persistent

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>