doh2: C implementation of DoH using nghttp2

TODO:

• update documentation
• update packaging
• tests
• allow compilation with GnuTLS >= 3.3.29 && < 3.5.0 (missing GNUTLS_ALPN_MANDATORY)
• check darwin compilation and errors (strchnul missing?)
• make libnghttp2 optional dependency
• update base64url from Knot
• create issue documenting unimplemented features
• mark lua-http doh obsolete and add warning?
• improve protocol mismatch handling (TLS, HTTP/1) nghttp2_session_mem_recv failed: Received bad client magic byte string (-903)
• disallow use of both doh and doh2 on the same port (#615)

Fixes: #600 (closed)