Implemented the "visibility metaphor"

Under the “visibility metaphor”, values of the isMemberOf attribute
that are DNs of groups that the user is not authorized to see are made
invisible. This means removing them from the list of returned attribute
values, making sure that compare operations always returned FALSE for
non-authorized isMemberOf values, and making sure that the use of
non-authorized isMemberOf values in search filters does not result in
information being leaked.