Previously if HTTP/2 traffic is appended to HTTP Upgrade response header
(thus they are in the same buffer), the trailing HTTP/2 traffic is not
processed and lost. The appended data is most likely SETTINGS frame.
If it is lost, nghttp2 library complains server does not obey the HTTP/2
protocol and issues GOAWAY frame and curl eventually drops connection.
This commit fixes this problem and now trailing data is processed.
1 - fixes the warnings when built without http2 support
2 - adds CURLE_HTTP2, a new error code for errors detected by nghttp2
basically when they are about http2 specific things.
