Unhooks domdoc properties from both the domdoc and xmlDoc.
After the domdoc <-> xmlDoc connection is no longer an open question,
the domdoc::properties pointer can be removed and rerouted through
xmlDoc::_private::properties as long as it's impossible to have a domdoc
with domdoc::node.node == NULL.  With all of the checks for this, the
current code suggests that it's perfectly possible.