Tape Length Support

Situation

  The backup system doesn't know anything about tape lengths.
  Unfortunately, there are some hardware constraints that ought to be
  obeyed - older DAT drives can't handle anything larger than a 60m
  tape.  It would be nice if all-backups didn't choose inappropriate
  tapes for use in drives with limited abilities.

Implementation

  Add a length field to the entries in the tape database (which an
  appropriate default for each type).  This could be set in add-tape
  (by adding an extra option that allows you to override the default
  for this type of tape).

  Change the tape definitions in the backup.config file to allow one
  to specify a maximum tape length allowed for each drive.

  Change all-backups so that it won't choose inappropriate tapes for a
  given drive.

