But 8787: don't load the page if OpacMaintenance is ON

But 8787: don't load the page if OpacMaintenance is ON

If the syspref OpacMaintenance is ON, it is useless to loaded the
requested page.

To test:
- switch on the syspref OpacMaintenace
- check in your apache access log, zebra log, etc. the requested page is
  not loaded (i.e. on the opac-search.pl page)

Signed-off-by: Marc Veron <veron@veron.ch>
Checked by watching  other_vhosts_access.log
Works as expected

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>