Upon HTTP_1_1_REQUIRED, retry the request with HTTP/1.1

This is a companion patch to cbea2fd2c (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.

Note: we will still keep the NTLM workaround, as it avoids an extra
round trip.

Daniel Stenberg helped a lot with this patch, in particular by
suggesting to introduce the Curl_h2_http_1_1_error() function.

Closes #3349

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

lib/multi.c

@@ -46,6 +46,7 @@
 #include "vtls/vtls.h"
 #include "connect.h"
 #include "http_proxy.h"
+#include "http2.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
 #include "curl_memory.h"
@@ -1955,6 +1956,25 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           done = TRUE;
         }
       }
+      else if((CURLE_HTTP2_STREAM == result) &&
+                Curl_h2_http_1_1_error(data->easy_conn)) {
+        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 */
+          result = CURLE_OK;
+          done = TRUE;
+        }
+      }
 
       if(result) {
         /*