Bug 30101: Fix OPAC advanced search after Bug 29844

Bug 30101: Fix OPAC advanced search after Bug 29844

Bug 29844 forgot to add as_list to opac-search.pl.
You can see it changed search.pl

Patch fixes and also adds explicit interface = opac in get_search_groups() call.
Since intranet is explicit, opac should also be.
We keep opac as default value.

Test plan :
1) Create a libary group for OPAC and one for staff
2) Go to OPAC advanced search page
3) Check you see the OPAC libary group

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>