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.

docs/HTTP2.md

@@ -96,6 +96,9 @@ curl tool
 
 curl offers the `--http2` command line option to enable use of HTTP/2.
 
+curl offers the `--http2-prior-knowledge` command line option to enable use of 
+HTTP/2 without HTTP/1.1 Upgrade.
+
 Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.
 
 HTTP Alternative Services

docs/curl.1

@@ -150,6 +150,11 @@ version. (Added in 7.33.0)
 .IP "--http2"
 (HTTP) Tells curl to issue its requests using HTTP 2. This requires that the
 underlying libcurl was built to support it. (Added in 7.33.0)
+.IP "--http2-prior-knowledge"
+(HTTP) Tells curl to issue its requests using HTTP 2 without HTTP/1.1 Upgrade. 
+This requires prior knowledge that the server supports HTTP 2.
+This requires that the underlying libcurl was built to support it. 
+(Added in 7.49.0)
 .IP "--no-npn"
 Disable the NPN TLS extension. NPN is enabled by default if libcurl was built
 with an SSL library that supports NPN. NPN is used by a libcurl that supports

docs/libcurl/symbols-in-versions

@@ -695,6 +695,7 @@ CURL_HTTP_VERSION_1_1           7.9.1
 CURL_HTTP_VERSION_2             7.43.0
 CURL_HTTP_VERSION_2_0           7.33.0
 CURL_HTTP_VERSION_2TLS          7.47.0
+CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE 7.49.0
 CURL_HTTP_VERSION_NONE          7.9.1
 CURL_IPRESOLVE_V4               7.10.8
 CURL_IPRESOLVE_V6               7.10.8