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.
sasl: Post DIGEST-MD5 SSPI code tidy up

* Added comments to SSPI NTLM message generation
* Added comments to native DIGEST-MD5 code
* Removed redundant identity pointer
sasl: Corrected add of Curl_sasl_decode_digest_md5_message() from 2c49e96092
sasl: Corrected missing free of decoded challenge message from 607883f13c
sasl: Fixed compilation warning in SSPI builds

warning: 'sasl_digest_get_key_value' defined but not used
sasl: Added curl_memory.h include as per test 1132
sasl: Fixed compilation warning

warning: no previous prototype for 'Curl_sasl_create_digest_md5_message'
tests: Disabled DIGEST-MD5 tests when running with SSPI enabled
RELEASE-NOTES: Synced with 5cdb61abb2
sasl: Added forward declaration of structures following recent changes

To avoid urldata.h being included from the header file or that the
source file has the correct include order as highlighted by one of
the auto builds recently.