If gss_inquire_attrs_for_mech() is called for a mechanism which does
not implement it, the call will succeed with mech_attrs set to
GSS_C_NO_OID_SET (as is explicitly allowed by RFC 5587).
generic_gss_test_oid_set_member() returns an error on this value,
causing gss_accept_sec_context() to erroneously deny the mechanism
when no verifier credential handle is supplied. Change
allow_mech_by_default() to explicitly check for no mech attribute set.
(cherry picked from commit bd321c9caa6dc4b034bc3279a1af39af4c41210d)