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.

docs/libcurl/symbols-in-versions

@@ -671,6 +671,7 @@ CURL_GLOBAL_WIN32               7.8.1
 CURL_HTTP_VERSION_1_0           7.9.1
 CURL_HTTP_VERSION_1_1           7.9.1
 CURL_HTTP_VERSION_2_0           7.33.0
+CURL_HTTP_VERSION_2             7.43.0
 CURL_HTTP_VERSION_NONE          7.9.1
 CURL_IPRESOLVE_V4               7.10.8
 CURL_IPRESOLVE_V6               7.10.8

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
  */