Bug 28337: Add System-control-number index to Zebra MARC21 files

Bug 28337: Add System-control-number index to Zebra MARC21 files

To test:
1 - Apply patch
2 - Copy zebra files to destination:
    cp /kohadevbox/koha/etc/zebradb/marc_defs/marc21/authorities/authority-koha-indexdefs.xml /etc/koha/zebradb/marc_defs/marc21/authorities/authority-koha-indexdefs.xml
    cp /kohadevbox/koha/etc/zebradb/marc_defs/marc21/authorities/authority-zebra-indexdefs.xsl /etc/koha/zebradb/marc_defs/marc21/authorities/authority-zebra-indexdefs.xsl
3 - Reindex authorities
4 - Edit an authority and add 035$aExpialodocious
5 - Export the authority
6 - Create a new record matchign rule:
    Matching rule code: OCN
    Description: Other control number
    Match threshhold: 1000
    Record type: Authority record
    Search-index: Other-control-number
    Score: 1000
    Tag: 035
    Subfields: a
7 - Stage the record and use the new matchign rule
8 - Match found!

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>