Bug 13287: Add a system preference to define the number of days used in purge_suggest...

Bug 13287: Add a system preference to define the number of days used in purge_suggestions.pl

    The system preferences value is used whenever purge_suggestions.pl is called without the 'days' parameter.
    This patch uses the preference description suggested by comment #9.
    This version should now be cleanly applicable.

    I Apply the patch
    II Run updatedatabase.pl

    a) Run purge_suggestions.pl without the days parameter
       - validate that there is an error message
    b) Run purge_suggestions.pl with the days parameter
       - validate that there is no error message
    c) Insert a number of days in the system variable PurgeSuggestionsOlderThan
    d) Run purge_suggestions.pl without the days parameter
       - validate that there is no error message

Signed-off-by: Liz Rea <liz@catalyst.net.nz>
Tested per plan, all tests pass.

Signed-off-by: Marc Veron <veron@veron.ch>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>