pfctl: clear statistic for specified addresses

The ioctl DIOCRCLRASTATS provides the functionality of clearing stats
not only for the whole table for for addresses stored in that table. The
functionality was missing from pfctl, though. Add it now.

PR: 282877
Obtained from: OpenBSD, kirill <kirill@openbsd.org>, e496dff3a7
MFC after: 3 weeks