Brioche to-do list, in random order.

* No need to keep undo directory if FTP backup is used.
* Better yet, don't keep any archive and snar locally, except the full.snar if
  FTP backup is used.
* Usage of FTP could be specified in briochetab for each backup.
* Snapshot size specified in briochetab, or automatically determined.
* Implement hooks, in various parts of the script, that would allow to run
  external commands (for instance, to freeze a database just before making a
  snapshot).
* Support GnuPG to encrypt the files