ntlm_sspi: fix authentication using Credential Manager

If you pass empty user/pass asking curl to use Windows Credential
Storage (as stated in the docs) and it has valid credentials for the
domain, e.g.
curl -v -u : --ntlm example.com
currently authentication fails.
This change fixes it by providing proper SPN string to the SSPI API
calls.

Fixes https://github.com/curl/curl/issues/1622
Closes https://github.com/curl/curl/pull/1660