pf (t)ftp-proxy: use libpfctl instead of 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/D41647

(cherry picked from commit ddd08375c85576b49fb9a34968ba2c2f4f8d56cf)