many changes introducing the new max_depth feature in list_dir(), and it works just like GNU find does.  The badly named "max_dives()" method has been renamed to "abort_depth" bot for clarity and in order to avoid confusion with the new "max_depth" feature.  Full backward compat for max_dives() is there.  All documentation updated to reflect the change.  True runaway recursion detection (via an inodes-seen hash table) now in place as well.  test suite updated to account for all changes