Reorganization: ripped util.py to shreds, creating in the process:

- file_util.py: operations on single files
  - dir_util.py: operations on whole directories or directory trees
  - dep_util.py: simple timestamp-based dependency analysis
  - archive_util.py: creation of archive (tar, zip, ...) files
The functions left in util.py are miscellany that don't fit in any of the
new files.