* add HTTP/2-only supporting providers: Mullvad, Digitale-Gesellschaft, dns.sb and Rubyfish.cn
* switch default provider from Google to Cloudflare
* add IPv6 addresses for bootstrap resolvers for Google DNS
* add secondary bootstrap resolver (Cloudflare's) to all providers with a single bootstrap resolver
* modify model/cbi file to show HTTP/2-only providers (and help texts) on HTTP/2-supporting systems

Signed-off-by: Stan Grishin <stangri@melmac.net>