fix p.security.ACLPermitsResult to subclass p.security.PermitsResult

The ``IAuthorizationPolicy`` is expected to return an instance of
``PermitsResult`` and the ``ACLPermitsResult`` now subclasses this to
form a consistent class hierarchy.

Similarly the ``ACLDenied`` subclasses ``Denied`` and ``ACLAllowed``
subclasses ``Allowed`` for consistency.