Upon HTTP_1_1_REQUIRED, retry the request with HTTP/1.1

This is a companion patch to cbea2fd (NTLM: force the connection to
HTTP/1.1, 2018-12-06): with NTLM, we can switch to HTTP/1.1
preemptively. However, with other (Negotiate) authentication it is not
clear to this developer whether there is a way to make it work with
HTTP/2, so let's try HTTP/2 first and fall back in case we encounter the
error HTTP_1_1_REQUIRED.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

lib/multi.c

@@ -1948,6 +1948,24 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         else if(!result)
           result = ret;
 
+        if(retry) {
+          /* if we are to retry, set the result to OK and consider the
+             request as done */
+          result = CURLE_OK;
+          done = TRUE;
+        }
+      } else if(CURLE_HTTP2_STREAM == result && data->set.errorbuffer &&
+                strstr(data->set.errorbuffer, "HTTP_1_1_REQUIRED")) {
+        CURLcode ret = Curl_retry_request(data->easy_conn, &newurl);
+
+        infof(data, "Forcing HTTP/1.1 for NTLM");
+        data->set.httpversion = CURL_HTTP_VERSION_1_1;
+
+        if(!ret)
+          retry = (newurl)?TRUE:FALSE;
+        else
+          result = ret;
+
         if(retry) {
           /* if we are to retry, set the result to OK and consider the
              request as done */