extensions: libip6t_SNAT: Add translation to nft

extensions: libip6t_SNAT: Add translation to nft

Add translation for SNAT target to nftables.

Examples:

$ sudo ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:80
nft add rule ip6 nat postrouting oifname eth0 ip6 nexthdr tcp counter snat fec0::1234 :80

$ sudo ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:1-20
nft add rule ip6 nat postrouting oifname eth0 ip6 nexthdr tcp counter snat fec0::1234 :1-20

$ sudo ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:123 --random
nft add rule ip6 nat postrouting oifname eth0 ip6 nexthdr tcp counter snat fec0::1234 :123 random

$ sudo ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:123 --random-fully --persistent
nft add rule ip6 nat postrouting oifname eth0 ip6 nexthdr tcp counter snat fec0::1234 :123 fully-random,persistent

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