Bug 13346: OpacExportOptions becomes multiple

Bug 13346: OpacExportOptions becomes multiple

The list of options is limited, the interface could be more ergonomic.

Test plan:
- verify that you are still allowed to change (add/remove) options for the
  OpacExportOptions pref.
- At the OPAC, verify the selected options appear correctly (check the
  3 pages opac-ISBDdetail.pl, opac-MARCdetail.pl and opac-detail.pl).

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>