Bug 8550: restore ISSN field to Z39.50 search

Bug 8550: restore ISSN field to Z39.50 search

Sometime in 2009 or so, the ISSN field was removed from the Z39.50 search
page, and the ISBN field replaced with a combined ISBN/ISSN field. Since
this breaks ISBN search for most Z39.50 targets (due to the
differing treatment of ISBN vs. ISSN), this is a bug not a feature.

A future enhancement would be to use Business::ISBN to search for both
ISBN10 and ISBN13.

To test:
1) Apply patch.
2) Do a search for an ISBN using the ISBN field.
3) Do a search for an ISSN using the ISSN field.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>