Add gss_inquire_attrs_for_mech()
    
    Since commit 030a4a03a ("Report inquire_attrs_For_mech mech failures") in
    MIT krb5, NTLMSSP fallback within SPNEGO is no longer working. It seems
    that providing a gss_inquire_attrs_for_mech() function is now mandatory.
    
    Although it does seem that perhaps krb5 should be a little more forgiving
    and just assume GSS_C_NO_OID_SET, fix it anyway.
    
    Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>