Bug 10281: searching for a search domain fails

For reasons I cannot fathom, the split() in handling multi-branch
limits was not coming up with a valid search group code. Replacing
the split() with a substr() and creating the CGI parameter as a string
rather than as an arrayref fixes the problem. This problem may not
affect all installations, since I tested this exact feature just under
two months ago and it worked fine, and none of the relevant code has
been changed since then that I can see.

To test:
1) Create search group, and add at least one library to it, in
   /cgi-bin/koha/admin/branches.pl
2) Apply patch
3) Try doing a search limited to your search group, making sure that
   the search will match items that belong to a library in the search
   group
4) Sign off