general purpose class for handling Access Control List.
Could be used for a new config system, IMAP_ACL, controlling treeviews,
filemanagers, and whatever we can come up with.

The class contains no vars and is strickly a utility class. The acllist are
arrays so we do not need to include the class before the session. We only
need to include it when we actually want to do something with acllists.