Bug 4839 (System pref for MARC field 008 authorities; additional logic for position...

Bug 4839 (System pref for MARC field 008 authorities; additional logic for position 14) [version 3.0.x]

Currently, the default for this marc field is hardcoded in the plugin code of
marc21_field_008_authorities.pl (two occurrences).
I suggest to move this value to a new system preference. If this pref should
not exist, the old hardcoded value could still be used.

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).

This patch applies to 3.0.x only. Additional patch submitted for master.