curl.h: add CURL_HTTP_VERSION_2

The protocol is named "HTTP/2" after all. It is an alias for the
existing CURL_HTTP_VERSION_2_0 enum.

include/curl/curl.h

@@ -1695,6 +1695,11 @@ enum {
   CURL_HTTP_VERSION_LAST /* *ILLEGAL* http version */
 };
 
+/* Convenience definition simple because the name of the version is HTTP/2 and
+   not 2.0. The 2_0 version of the enum name was set while the version was
+   still planned to be 2.0 and we stick to it for compatibility. */
+#define CURL_HTTP_VERSION_2 CURL_HTTP_VERSION_2_0
+
 /*
  * Public API enums for RTSP requests
  */