Add support for showing ipv6 NAT table in Luci

When kmod-nf-nat6 and kmod-ipt-nat6 are installed, the firewall has also
the 'nat' table for ipv6, and packages like 'adblock' utilize that table.

Currently that table is not shown on the Luci firewall status page,
although it is visible by 'ip6tables -L -v -t nat' from console.

Detect 'nat' table's presence from /proc/net/ip6_tables_names

Show 'nat' table in Status->Firewall->IPv6 if that table is present.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>