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.

src/tool_getparam.c

@@ -184,6 +184,7 @@ static const struct LongShort aliases[]= {
   {"0",   "http1.0",                 FALSE},
   {"01",  "http1.1",                 FALSE},
   {"02",  "http2",                   FALSE},
+  {"03",  "http2-prior-knowledge",   FALSE},
   {"1",  "tlsv1",                    FALSE},
   {"10",  "tlsv1.0",                 FALSE},
   {"11",  "tlsv1.1",                 FALSE},
@@ -1036,6 +1037,10 @@ ParameterError getparameter(char *flag,    /* f or -long-flag */
         /* HTTP version 2.0 */
         config->httpversion = CURL_HTTP_VERSION_2_0;
         break;
+      case '3':
+        /* HTTP version 2.0 over clean TCP*/
+        config->httpversion = CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE;
+        break;
       }
       break;
     case '1': /* --tlsv1* options */

src/tool_help.c

@@ -111,6 +111,7 @@ static const char *const helptext[] = {
   " -0, --http1.0       Use HTTP 1.0 (H)",
   "     --http1.1       Use HTTP 1.1 (H)",
   "     --http2         Use HTTP 2 (H)",
+  "     --http2-prior-knowledge  Use HTTP 2 without HTTP/1.1 Upgrade (H)",
   "     --ignore-content-length  Ignore the HTTP Content-Length header",
   " -i, --include       Include protocol headers in the output (H/F)",
   " -k, --insecure      Allow connections to SSL sites without certs (H)",