vauth: Introduced Curl_auth_is_<mechansism>_supported() functions

As Windows SSPI authentication calls fail when a particular mechanism
isn't available, introduced these functions for DIGEST, NTLM, Kerberos 5
and Negotiate to allow both HTTP and SASL authentication the opportunity
to query support for a supported mechanism before selecting it.

For now each function returns TRUE to maintain compatability with the
existing code when called.