Bug 28965: filter_by_public() and filter_by_readable()

Bug 28965: filter_by_public() and filter_by_readable()

This patch adds helper methods for lists resultsets. Tests are added for
their behavior.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Virtualshelves.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>