Bug 24272: add check_sysprefs_cache.pl

Bug 24272: add check_sysprefs_cache.pl

This script ensure that no sysprefs have been changed directly in the database and/or
that the cache has not become corrupted. We have occasionally seen this happen on production sites

To test:
1 - In the staff interface go to Administration
2 - Search for system preference 'IntranetUserJS'
3 - Add content to the syspref:
    console.log('Hi!');
4 - On the command line launch mysql
    sudo koha-mysql kohadev
5 - Alter the syspref directly
    UPDATE systempreferences SET value = "console.log('Bye!');" WHERE variable = 'IntranetUserJS';
6 - run the script
    perl misc/maintenance/check_syspref_cache.pl
7 - You are warned about the altered system preference

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>