Bug 7818: -x option of rebuild_zebra.pl now works with DOM filter

Bug 7818: -x option of rebuild_zebra.pl now works with DOM filter

One consequence is that the -x and -a options are no longer
mutually exclusive.

Also, because of the way that the GRS-1 SGML filter works, if you're
indexing multiple documents, you can't just wrap them in a document
element, but the DOM filter *requires* it.  Consequently, two
new config settings in koha-conf.xml are added to indicate the
Zebra filter in use so that the -x option of rebuild_zebra.pl
knows whether to wrap the exported records or not:

- bib_index_mode (defaults to 'grs1' if not specified)
- auth_index_mode (defaults to 'dom')

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>