curl.h: add CURL_VERSION_HTTP2 as a feature

It isn't added as a separate protocol as HTTP2 will be done over HTTP://
URLs that can be upgraded to HTTP2 if the server supports it as well.
curl -V: output HTTP2 as a feature if present
http2: add http2.[ch] and add nghttp2 version output
Curl_setopt: refuse CURL_HTTP_VERSION_2_0 if built without support
curl: add --http1.1 and --http2.0 options