Helper function, descending from the given root_node to the requested_path specified.
Returns HTTP Responses or Status Codes.
If the path points to a regular file, it serves its contents.
If the path points to a symlink, it sends a redirect to the target (pretending base_path, if relative)
If the path points to a directory, files of index_names are tried,
if no files matched then a directory listing is returned if auto_index is enabled.