RepoInitGrammer: Add repository-level marker to pathsList

In order to be able to cover repository level access control entrieswith the PrincipalAccessControlList (see SLING-8602), i would like to expand the RepoInitGrammer.pathsList() such that it not only handles absolute paths but in addition a marker for the repository level permissions (in JCR marked with a null path).

essentially this would also create an alternative way to edit policies for the null path (today covered with set repository ACL ...).

will provide a proposed patch and tests later this week.