Bug 8746: rebuild_zebra_sliced.sh now export/index records as MARCXML

Bug 8746: rebuild_zebra_sliced.sh now export/index records as MARCXML

This avoid indexing failures due to "bad offset" or "bad length" error
with ISO2709 format

+ minor improvements:
  -  --length parameter is optional. If not given, it will execute the
     right sql query to find the number of records to index
  -  new parameter --reset-index. If set, index is reset before indexing

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Comment: Work as described. No errors.

Test: Edit record to make it longer than 9999. Without patch rebuild_sliced
fails. With patches works.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>