Bug 32960: Add option "not checked out" to item search

Bug 32960: Add option "not checked out" to item search

This enhances the availability search option on item search
to have a third option: ignore, checked out and not checked out.
I chose 'not checked out' as available could be misleading,
the item might still be lost or withdrawn and not on the shelf.

To test:
* Apply patch
* Make sure some items are checked out
* Search with 'ignore' - note number of results
* Search with 'checked out' - note number of results
* Search with 'not checked out' - number of results should
  match ignore minus checked out

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>