extensions: libip6t_LOG: Add translation to nft

extensions: libip6t_LOG: Add translation to nft

Add translation for options log-level and log-prefix of LOG target
to nftables.
Full translation of this target awaits the support for the options
log-tcp-sequence, log-tcp-options, log-ip-options, log-uid and
log-macdecode in nftables

Examples:

$ sudo ip6tables-translate -A FORWARD -p tcp -j LOG --log-level debug
nft add rule ip6 filter FORWARD ip6 nexthdr tcp counter log level debug

$ sudo ip6tables-translate -A FORWARD -p tcp -j LOG --log-prefix "Checking log"
nft add rule ip6 filter FORWARD ip6 nexthdr tcp counter log prefix \"Checking log\" level warn

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