http2: support "prior knowledge", no upgrade from HTTP/1.1

Supports HTTP/2 over clear TCP

- Optimize switching to HTTP/2 by removing calls to init and setup
before switching. Switching will eventually call setup and setup calls
init.

- Supports new version to “force” the use of HTTP/2 over clean TCP

- Add common line parameter “--http2-prior-knowledge” to the Curl
  command line tool.

include/curl/curl.h

@@ -1727,6 +1727,8 @@ enum {
   CURL_HTTP_VERSION_1_1,  /* please use HTTP 1.1 in the request */
   CURL_HTTP_VERSION_2_0,  /* please use HTTP 2 in the request */
   CURL_HTTP_VERSION_2TLS, /* use version 2 for HTTPS, version 1.1 for HTTP */
+  CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE,  /* please use HTTP 2 without HTTP/1.1
+                                           Upgrade */
 
   CURL_HTTP_VERSION_LAST /* *ILLEGAL* http version */
 };