extensions: libxt_multiport: Add translation to nft

extensions: libxt_multiport: Add translation to nft

Add translation for multiport to nftables, which it's supported natively.

Examples:

$ sudo iptables-translate -t filter -A INPUT -p tcp -m multiport --dports 80,81 -j ACCEPT
nft add rule ip filter INPUT ip protocol tcp tcp dport { 80,81} counter accept

$ sudo iptables-translate -t filter -A INPUT -p tcp -m multiport --dports 80:88 -j ACCEPT
nft add rule ip filter INPUT ip protocol tcp tcp dport 80-88 counter accept

$ sudo iptables-translate -t filter -A INPUT -p tcp -m multiport ! --dports 80:88 -j ACCEPT
nft add rule ip filter INPUT ip protocol tcp tcp dport != 80-88 counter accept

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