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