bug 2986: tweak authtype index for MARC21 (GRS-1)

bug 2986: tweak authtype index for MARC21 (GRS-1)

Following a similar patch for UNIMARC, tweak the
authtype index for MARC21 authorities if the GRS-1
Zebra filter is in use.

Note that it is recommended that *DOM* mode indexing
be used for MARC21 authorities; if you're using DOM mode,
it is not necessary to rebuild the index.  However, if
you're using the GRS-1 definitions (record.abs), it will
be necessary to reindex the authority records using

misc/migration_tools/rebuild_zebra.pl -a -r

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>