pfctl: use libpfctl to retrieve pf status

Rather than call DIOCGETSTATUS ourselves use the new libpfctl functions.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31697

(cherry picked from commit 80078d9d38fde6f146de28809640b2c7bff45a6c)