extensions: libxt_udp: add translation to nft

Some examples:

 $ sudo iptables-translate -A INPUT -p udp -i eth0 --sport 53 -j ACCEPT
add rule ip filter INPUT iifname eth0 udp sport 53 counter accept

 $ sudo ./iptables-translate -A OUTPUT -p udp -o eth0 --dport 53:66 -j DROP
add rule ip filter OUTPUT oifname eth0 udp dport 53-66 counter drop

 $ sudo ./iptables-translate -I OUTPUT -p udp -d 8.8.8.8 -j ACCEPT
nft insert rule ip filter OUTPUT ip protocol udp ip daddr 8.8.8.8 counter accept

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
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