Bug 4839: Adjust default values for MARC21 authority Fixed-Length Data Elements

Bug 4839: Adjust default values for MARC21 authority Fixed-Length Data Elements

Currently, the default for this MARC control field is hardcoded in the plugin code of marc21_field_008_authorities.pl (two occurrences).
This patch moves it into a new syspref. If this pref should not exist, the old hardcoded value is used. Authorities preference file is updated too.
A warning is generated if the length of the field is not correct.

Furthermore, the default for position 14 (heading use: main or added entry)
should be dependent on the authority type. For CHRON_TERM, GENRE/FORM and
TOPIC_TERM, it should be b (not appropriate) instead of a (appropriate).

Rebased on March 17. Follow-up patch added for installation pref.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>