http: do not require a user name when using CURLAUTH_NEGOTIATE

http: do not require a user name when using CURLAUTH_NEGOTIATE

In order to get Negotiate (SPNEGO) authentication to work in HTTP you
used to be required to provide a (fake) user name (this concerned both
curl and the lib) because the code wrongly only considered
authentication if there was a user name provided, as in:

  curl -u : --negotiate https://example.com/

This commit leverages the `struct auth` want member to figure out if the
user enabled CURLAUTH_NEGOTIATE, effectively removing the requirement of
setting a user name both in curl and the lib.

Signed-off-by: Marin Hannache <git@mareo.fr>
Reported-by: Enrico Scholz
Fixes https://sourceforge.net/p/curl/bugs/440/
Fixes #1161
Closes #9047