sasl_sspi: Tell the server we don't support a GSSAPI receive buffer
docs: Added Kerberos V5 to the --user SSPI current credentials usage
docs: Added Kerberos V5 and NTLM domain information to --user
CURLOPT_USERPWD.3: Updated following Kerberos V5 SSPI changes

Added information about Kerberos V5 requiring the domain part in the
user name.

Mentioned that the user name can be specified in UPN format, and not
just in Down-Level Logon Name format, following the information
added in commit 7679cb3fa8 reworking the exisitng information in the
process.
CURLOPT_USERNAME.3: Added Kerberos V5 and NTLM domain information

This repeats what has already been documented in both the curl manpage
and CURLOPT_USERPWD documentation but is provided here for completeness
as someone may not especially read the latter when using libcurl.