pf: Introduce nvlist variant of DIOCGETSTATUS

Make it possible to extend the GETSTATUS call (e.g. when we want to add
new counters, such as for syncookie support) by introducing an
nvlist-based alternative.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31694

(cherry picked from commit 2b10cf85f8684f822511d7b9377e256ab623abbc)