Accept GSS mechs which don't supply attributes

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.

ticket: 8840 (new)
tags: pullup
target_version: 1.17-next
target_version: 1.16-next