extensions: libip6t_REJECT: Add translation to nft

extensions: libip6t_REJECT: Add translation to nft

Add translation for target REJECT to nftables.

Examples:

$ sudo ip6tables-translate -A FORWARD -p TCP --dport 22 -j REJECT --reject-with icmp6-reject-route
nft add rule ip6 filter FORWARD tcp dport 22 counter reject with icmpv6 type reject-route

$ sudo ip6tables-translate -A FORWARD -p TCP --dport 22 -j REJECT --reject-with tcp-reset
nft add rule ip6 filter FORWARD tcp dport 22 counter reject with tcp reset

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