Add a new parameter: psz_flatpath. If not NULL, readdir_helper_additem() will
un-flatten the folder hierarchy. It will create an input item for each sub
folders (separated by '/') of psz_nullpath.

Example: a stream directory that have the following items:
 - "foo/bar/item1"
 - "foo/bar/item2"

will output the following items while using this new parameter:
 - foo
   - bar
     - item1
     - item2

This functionality can be used by "stream directory" modules that have a
flattened folder hierarchy, like libarchive.

Access modules should not need this functionality.