Bug 28847: (follow-up) Handle branch_group_limit

Bug 28847: (follow-up) Handle branch_group_limit

To test:
1 - Create a library group as an OPAC search group
2 - Enable OpacAddMastheadLibraryPulldown
3 - Browse to:
    http://localhost:8080/cgi-bin/koha/opac-search.pl?idx=&q=t&branch_group_limit=multibranchlimit-1&weight_search=1
4 - Confirm search works and returns correct results
5 - Confirm dropdown is correctly populated

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>