Added a new API to ``OFS.metaconfigure.get_registered_packages`` which replaces any direct access to ``Products._registered_packages``.

Changed product install so it won't write persistent changes only to abort them. Instead we don't make any database changes in the first place.

Directly extend and use the Zope Toolkit KGS release 1.0dev-r113891 from http://download.zope.org/zopetoolkit/index/. We should get a proper alpha release in time.

Also makes the intent clearer.

Changed the default for ``enable-product-installation`` to off. This matches the default behavior of buildout installs via plone.recipe.zope2instance.

- LP #597594: Performance optimization in OFS.subscriber.maybeWarnDeprecated.

``ZPublisher.interfaces.UseTraversalDefault`` to indicate that there is no
special casing for the given name and that standard traversal logic should be
applied.