Mark states with what is essentially a second label to make it easy to
kill them all.
The intended use case is to allow certain hosts access based on a
schedule (for which userspace must add/remove the pass/block rule
according to the desired schedule).
However, this doesn't affect established connections (which could
potentially be long-lived and used to bypass the schedule policy).
The 'schedule' keyword allows these states to be marked, and terminated.
Obtained from: pfSense
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")