Prepare for RFN support:

Prepare for RFN support:
- maintain RFN tree
- files with catalog entry name as content
- sanity checks for "foreign" RFN:
  - avoid overwriting (=no duplicitis of RFN)
  - check content before deleting (=not deleting RFN of other catalog entry)
- update RFN tree in rename() and unlink() calls
- cleanup empty directories