sasl: Added support for DIGEST-MD5 via Windows SSPI
sasl: Corrected pre-processor inclusion of SSPI based DIGEST-MD5 code

When CURL_DISABLE_CRYPTO_AUTH is defined the DIGEST-MD5 code should not
be included, regardless of whether USE__WINDOWS_SSPI is defined or not.
This is indicated by the definition of USE_HTTP_NEGOTIATE and USE_NTLM
in curl_setup.h.