Bug 19451: Add no_overwrite option to borrowers-force-messaging-defaults.pl

Bug 19451: Add no_overwrite option to borrowers-force-messaging-defaults.pl

This option allows you to add preferences only when they are not yet
present. In other words: skip patrons that already set their prefs.

Test plan:
[1] Delete all borrower messaging prefs for a patron.
[2] Run borrowers-force-messaging-defaults.pl -no-overwrite -doit
    Verify that the patron now has default msg preferences.
[3] Change his settings and make them non-default.
    For instance, increase days in advance.
[4] Run borrowers-force-messaging-defaults.pl -no-overwrite -doit
    Verify that the patron still has the non-default settings.
[5] Run borrowers-force-messaging-defaults.pl -doit
    Verify that the patron msg prefs have been overwritten.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>