Merge branch 'doh-client'.
This adds experimental support for resolving names using DNS over HTTPS (DoH) for the Origin request mechanism. To enable it, you must provide the client with an `--origin-doh-base` option and give it a DoH base URL argument (like `https://mozilla.cloudflare-dns.com/dns-query` as used by Mozilla, more options [here](https://github.com/curl/curl/wiki/DNS-over-HTTPS)). DoH requests are handled internally by the client and follow the usual request routes, so they heed public/private browsing differences and enabled/disabled request mechanisms. This means that requests sent in private browsing mode can use the Origin and Proxy mechanisms (thus staying private), and requests sent in public mode can use Origin, Injector or Cache (and thus get retrieved from others or cached and shared). Separate DoH requests for both IPv4 and IPv6 addresses are always performed. Entries cached by other clients can be fetched as long as they use the same DoH base URL, IP version and stick to the minimal DNS query message (so that they `GET` URLs match). **Important note:** Results are not yet privately cached (beyond the distributed cache), which means that each requested URL triggers a full DoH request, which makes browsing quite slow.