extensions: libip6t_ah: Add translation to nft

extensions: libip6t_ah: Add translation to nft

Add translation for AH to nftables.

Examples:

$ sudo ip6tables-translate -A INPUT -m ah --ahspi 500 -j DROP
nft add rule ip6 filter INPUT ah spi 500 counter drop

$ sudo ip6tables-translate -A INPUT -m ah --ahspi 500:550 -j DROP
nft add rule ip6 filter INPUT ah spi 500-550 counter drop

$ sudo ip6tables-translate -A INPUT -m ah ! --ahlen 120
nft add rule ip6 filter INPUT ah hdrlength != 120 counter

$ sudo ip6tables-translate -A INPUT -m ah --ahres
nft add rule ip6 filter INPUT ah reserved 1 counter

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