extensions: libxt_dscp: Add translation to nft

extensions: libxt_dscp: Add translation to nft

Add translation for dscp to nftables, for both ipv4 and ipv6.

Examples:

$ sudo iptables-translate -t filter -A INPUT -m dscp --dscp 0x32 -j ACCEPT
nft add rule ip filter INPUT ip dscp 0x32 counter accept

$ sudo ip6tables-translate -t filter -A INPUT -m dscp ! --dscp 0x32 -j ACCEPT
nft add rule ip6 filter INPUT ip6 dscp != 0x32 counter accept

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