pfctl: Use DIOCGETSTATESNV

Migrate to using the new nvlist-based DIOCGETSTATESNV call to obtain the
states list.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30244

(cherry picked from commit bc941291473d8a2164e4ffc3f3e7e6a356cbe747)