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

tests/data/test2056

@@ -47,7 +47,7 @@ LD_PRELOAD=%PWD/libtest/.libs/libstubgss.so
 CURL_STUB_GSS_CREDS="KRB5_Alice"
 </setenv>
 <command>
--u: --negotiate http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+--negotiate http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>
 

tests/data/test2057

@@ -63,7 +63,7 @@ LD_PRELOAD=%PWD/libtest/.libs/libstubgss.so
 CURL_STUB_GSS_CREDS="NTLM_Alice"
 </setenv>
 <command>
--u: --negotiate http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+--negotiate http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>
 

tests/data/test2077

@@ -29,7 +29,7 @@ GSS-API
 curl --fail --negotiate to unauthenticated service fails
 </name>
 <command>
-http://%HOSTIP:%HTTPPORT/%TESTNUMBER -u : --fail --negotiate
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --fail --negotiate
 </command>
 </client>
 

tests/data/test2078

@@ -29,7 +29,7 @@ GSS-API
 curl --negotiate should not send empty POST request only
 </name>
 <command>
-http://%HOSTIP:%HTTPPORT/%TESTNUMBER -u : --negotiate --data name=value
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --negotiate --data name=value
 </command>
 </client>