Bug 13629: SingleBranchMode removes both library and availability search from advance...

Bug 13629: SingleBranchMode removes both library and availability search from advanced search

This patch modifies the OPAC advanced search form so that it is possible
to limit searches to available items even if there is only one library.

To test, apply the patch and go to the advanced search page in the OPAC.

 - With only one library configured: There should be a section labeled
   "Availability" with just the "only available" checkbox.

 - With multiple libraries: The section should be labeled "Location and
   availability" and should have the library dropdown and the
   availability checkbox.

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>