snmp_pf: use libpfctl's pfctl_get_status() rather than DIOCGETSTATUS

Prefer libpfctl functions over direct access to the ioctl whenever
possible. This will allow subsequent removal of DIOCGETSTATUS (in 15) as
there already is an nvlist-based alternative.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41650

(cherry picked from commit 6fbb9fbf7d659574512d706912e8fd0576b13573)