Better handling of new backup jobs, and more.

Guard against running two fetch-backup-work instances at the same
time, and have the operator confirm any changes it finds.
* fetch-backup-work: Atomically replace the list of backup tasks.
  Ask for confirmation before doing so.

Don't backup snes9x-rediffbackup.
* fetch-backup-work: Don't back up snes9x-rediffbackup, which
  seems to be an old backup in itself.

Back up ftp-pub and medreg.
* fetch-backup-work: Back up and / ftp-pub from hanna, and an
  sqldump of medreg from wrath.
* backup-one-task: wrath uses rdiff-backup stored in /usr/bin.

Added support for the --new option, which only runs backup jobs
that have never completed.
* backup-all: New option: --new.
* backup-one-task: New option: --new.