sasl_sspi: Allow DIGEST-MD5 to use current windows credentials

Fixed the ability to use the current log-in credentials with DIGEST-MD5.
I had previously disabled this functionality in commit 607883f13c as I
couldn't get this to work under Windows 8, however, from testing HTTP
Digest authentication through Windows SSPI and then further testing of
this code I have found it works in Windows 7.

Some further investigation is required to see what the differences are
between Windows 7 and 8, but for now enable this functionality as the
code will return an error when AcquireCredentialsHandle() fails.
sspi: Return CURLE_LOGIN_DENIED on AcquireCredentialsHandle() failure

Return a more appropriate error, rather than CURLE_OUT_OF_MEMORY when
acquiring the credentials handle fails. This is then consistent with
the code prior to commit f7e24683c4 when log-in credentials were empty.