Accept GSS mechs which don't supply attributes

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 bd321c9)

ticket: 8840
version_fixed: 1.17.1

src/lib/gssapi/mechglue/g_accept_sec_context.c

@@ -104,6 +104,10 @@ allow_mech_by_default(gss_OID mech)
     if (status)
 	return 0;
 
+    /* If the mechanism doesn't support RFC 5587, don't exclude it. */
+    if (attrs == GSS_C_NO_OID_SET)
+	return 1;
+
     /* Check for each attribute which would cause us to exclude this mech from
      * the default credential. */
     if (generic_gss_test_oid_set_member(&minor, GSS_C_MA_DEPRECATED,