ntlm: We prefer 'CURLcode result'
ntlm: Clean-up and standardisation of base64 decoding
sasl_sspi: Added missing RFC reference for HTTP Digest authentication
sasl: Moved Curl_sasl_gssapi_cleanup() definition into header file

Rather than define the function as extern in the source files that use
it, moved the function declaration into the SASL header file just like
the Digest and NTLM clean-up functions.

Additionally, added a function description comment block.
sasl_sspi: Corrected a couple of comment typos
TODO: Lets support QOP options in GSSAPI authentication