luci-app-firewall: allow ipv6 setup

Allow setup ipv6 for Port Forwards and NAT Rules if firewall4 is
used.

Add 'Restrict to address family' option for NAT Rules, if family is
any/empty , assume it is ipv4. this allow setup NAT6 rules in web ui

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>