- Implement IBM labels

- Implement EOF and EOV labels at the end of a volume.
- Fix a rather ugly problem with the PoolId not getting
  passed correctly. Now the DIR passes the Pool name and
  Media Type to the SD, who passes them back when requesting
  the next Volume. The DIR then looks up the correct PoolId.
  This takes more time, but always works, AND allows wild
  card Media Types (i.e. the SD can decide).
- The DIR <==> SD protocol has changed.