Bug 20100: Disallow access to superlib privileges at server side

Bug 20100: Disallow access to superlib privileges at server side

Depends on pref ProtectSuperlibPrivs.
If enabled, script member-flags.pl will not allow you to add or remove
superlib privs when you are no superlibrarian.
The follow-up patch will enable the check at client side.

Test plan:
[1] Enable the pref. Do not apply the third patch (client side).
[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 have an internal server error when
    you add or remove superlib privs. The log contains a warning.

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>