Bug 20100: Disallow access to superlibrarian privileges at client side

Bug 20100: Disallow access to superlibrarian privileges at client side

This last patch activates the check at client side.
If the pref ProtectSuperlibPrivs is enabled, non-superlibs should not be
able to change superlibrarian privileges via the interface.

Test plan:
[1] Enable the pref.
[2] Login as superlib and add/remove superlib privs to a staff user.
[3] Login as another user (no superlib, but having borrowers, permissions
    and staff_access). Verify that you cannot add or remove superlib
    privs.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: JM Broust <jean-manuel.broust@univ-lyon2.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>