Implemented a way to perform scan in a result set using Z39.50. This

Implemented a way to perform scan in a result set using Z39.50. This
is achieved by attaching the result set name in the characterInfo
(type InternationalString) of OtherInformation in the Scan Request PDU.
The result set is identified in the otherinformation by the new OID:
 USERINFO, Z3950_PREFIX.10.1000.81.4, "Scan-Set
This allows for scan in result set and faceted search . Zebra did some
of this in the APT term using attribute type 8 and value being result
set. Using the OtherInformation approach for this is cleaner and easier
to work with in proxies and the like. This facility can be used in
yaz-client using new command setscan which takes a result set as first