webstaff: add drop-down for item type Z39.50 search field

webstaff: add drop-down for item type Z39.50 search field

It should be noted that this works *only* for OCLC, as
not all targets support use attribute 1001, and the list
of values for that use attribute are target-defined.

A more general solution might be creating a new
config.z3950_attr_value_list table.

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