Implement local/html5fs/syncfs operations

We copy to/from these three file systems.  Ideally this should work just
fine, but it doesn't :(

* NaCl module can't see imported directories (even after restart).  This
  is the most confusing part.

* Why Javascript can't open persistent file system?  Why is it always
  temporary file system?  Did I get something wrong?

* If destination directory is not empty, you can't override it.
  (Because Chrome can't remove a directory with children by default?)