Moved duplicated dnsfilter code into its own function
dnsfilter: Do not force DNS resolution through the router if set to None, to allow client-defined DNS
Also moved the dnsfilter IPv6 configuration to its own function to reduce code size
DNSFilter: added two more user-defined servers; added option to force clients to use the nameserver provided to them by the router's DHCP server (or use the router itself if none was configured)