Add an /etc/periodic script to check for dports package vulnerabilities.

It's basically just 'pkg audit' at them moment, the output of which is
then included in the nightly security mails.

To disable this check for some reason, daily_status_dports_audit_enable
can be set to 'no' in /etc/periodic.conf.

Reported-by: joris
Revert "Add an /etc/periodic script to check for dports package vulnerabilities."

This reverts commit dced1188f0aae7a3a714a58540f8f6bf4043f35c.

Aw, actually there are scripts in /usr/local/etc/periodic doing just that.