NTLM: force the connection to HTTP/1.1

Since v7.62.0, cURL tries to use HTTP/2 whenever the server announces
the capability. However, NTLM authentication only works with HTTP/1.1,
and will likely remain in that boat (for details, see
https://docs.microsoft.com/en-us/iis/get-started/whats-new-in-iis-10/http2-on-iis#when-is-http2-not-supported).

When we just found out that we want to use NTLM, and when the current
connection runs in HTTP/2 mode, let's force the connection to be closed
and to be re-opened using HTTP/1.1.

Fixes https://github.com/curl/curl/issues/3341

.
Closes #3345

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