Bug 28943: Lower the risk of accidental patron deletion

Bug 28943: Lower the risk of accidental patron deletion

If you do not use a temporary self registration patron category,
you should actually make the preference
PatronSelfRegistrationExpireTemporaryAccountsDelay empty.

As the comment in sysprefs.sql already said, we should not let
a zero value in the pref delete patrons too.

The module is changed now, the test adjusted and
the description of both related sysprefs modified.

Test plan:
Run t/db_dependent/Members.t
Check in Administration the two adjusted OPAC pref descriptions.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>