extensions: libipt_LOG: Add translation to nft

extensions: libipt_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 iptables-translate -A FORWARD -p tcp -j LOG --log-level error
nft add rule ip filter FORWARD ip protocol tcp counter log level err

$ sudo iptables-translate -A FORWARD -p tcp -j LOG --log-prefix "Random prefix"
nft add rule ip filter FORWARD ip protocol tcp counter log prefix \"Random prefix\" level warn

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