bug 5825: (follow-up) consult item-level_itypes

bug 5825: (follow-up) consult item-level_itypes

This patch teaches GetHoldsQueueItems to consult
the item-level_itypes system preference and return
the item-level or bib-level item type accordingly.

To test:

- Arrange so that an item that shows up on the holds queue
  report has one item type while its bib has a different one.
- Run the report with item-level_itypes ON.  Verify that
  the item-level item type is displayed.
- Change item-level_itypes to OFF.  Run the report again and
  verify that the bib-level item type is displayed.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>