dnsfilter: Initial implementation (rc + build + rough webui)

DNS-based Filtering (dnsfilter) is based off Asus's YandexDNS implementation.
It allows the user to select LAN clients, and force them into using admin-defined
DNS services.  The supported services are:
 - OpenDNS
 - Norton ConnectSafe (all three levels)
 - YandexDNS (both levels)
DNSFilter: fix build errors; fix webui page reload after apply
DNSFilter: Fixed typo in list of supported DNS servers
DNSFilter: fix function declaration
Make Yandex and DNSFilter mutually exclusive, for simplicity (and since DNSFilter includes all of Yandex's functionality)
DNSFilter: implement default filtering support
DNSFilter: implement dnsfilter service action (for stop/starting)
Build: disable Yandex in favor of the new DNSFilter feature
DNSFilter: fix webui tabs
DNSFilter: Describe available services and provide links to provider's website on webui
webui: Increase DNSFilter limit to 64 clients
webui: Reject non-alphanum in DNSFilter name field
webui: Reject non-alphanum in YandexDNS name field