authpf: Start using libpfctl

Use pfctl_kill_states() rather than the DIOCKILLSTATES ioctl directly.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30057

(cherry picked from commit 2b2ed4a69730be72f792179d57dedf2945b3d1aa)