HAMMER - Enhance cleanup utility for HAMMER v3

HAMMER - Enhance cleanup utility for HAMMER v3

* For each PFS if the snapshots directory has not been overridden in
  the PFS config it will be migrated from <fs>/snapshots to
  /var/hammer/<path_from_root>.  The snapshots directory for the
  root directory itself will be called /var/hammer/root.  The old
  <fs>/snapshots directory will then removed.

  If a PFS has a snapshots directory config set no migration will
  occur.  Users can migrate it manually and then clear the snapshots
  directory in the PFS config if they desire.  We can't do it
  automatically because users might use this mechanic to mix snapshots
  with the live backup and needless to say cpdup()ing a live backup
  into /var is probably a bad idea.

* Fix a bug when generating softlinks in a directory that is not
  under the PFS being snapshotted.

* Add an optional third argument to the 'hammer snapshot' directive
  to allow a meta-data note to be specified.  The meta-data entry
  is created whether you specify a note or not.