digest_sspi: Handle 'stale=TRUE' directive in HTTP digest

- If the server has provided another challenge use it as the replacement
  input token if stale=TRUE. Otherwise previous credentials have failed
  so return CURLE_LOGIN_DENIED.

Prior to this change the stale directive was ignored and if another
challenge was received it would cause error CURLE_BAD_CONTENT_ENCODING.

Ref: https://tools.ietf.org/html/rfc2617#page-10

Bug: https://github.com/curl/curl/issues/928


Reported-by:  <tarek112@users.noreply.github.com>