Bug 16276: Make delete_patrons.pl deal with --last_seen

Bug 16276: Make delete_patrons.pl deal with --last_seen

This patch modify the delete_patrons.pl cronjob to deal with the
last_seen option.

To test it, you just have to use the --last_seen option and pass a valid
date (iso format)

Example:
  perl misc/cronjobs/delete_patrons.pl --last_seen="1984-02-04" --confirm
will delete all the patrons who do not have been active since this date.

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
https://bugs.koha-community.org/show_bug.cgi?id=12276

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>