Bug 11596: report warnings on the about patch if indexing options in koha-conf.xml...

Bug 11596: report warnings on the about patch if indexing options in koha-conf.xml  are missing

To test:
- Have a clean master install
- Verify that koha-conf.xml contains valid entries for:
  zebra_bib_index_mode and
  zebra_auth_index_mode
- Go to More > About Koha > System information
=> No "Warnings regarding the system configuration"
- Apply the patch
- Go to More > About Koha > System information
=> No "Warnings regarding the system configuration"
- Edit your koha-conf.xml file commenting one or both indexing mode
  entries.
- Go to More > About Koha > System information
=> You get "XML configuration file" warnings for each commented entry.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>