subtree is complete with no need to rehack its root. Simplifies a
bunch of code, and makes some utilities redundant.
* Using a single struct for all nodes, with subs=#f to mark files
* Names of struct is generic, no relation to actual files and dirs
* Added more tests
svn: r17527
empty directory is *not* dropped. This finally makes the results more
intuitive, and can now also express queries like only directories,
without their content. (See comments and new tests.)
svn: r17456
