pfctl: Fix clearing rules counters

After the migration to libpfctl for rule retrieval we accidentally lost
support for clearing the rules counters.

Introduce a get_clear variant of pfctl_get_rule() which allows rules
counters to be cleared.

MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29727