Upon HTTP_1_1_REQUIRED, retry the request with HTTP/1.1

This is a companion patch to cbea2fd2

 (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>