pf: Add DIOCGETSTATENV

Add DIOCGETSTATENV, an nvlist-based alternative to DIOCGETSTATE.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30242

(cherry picked from commit 1732afaa0dae9d844e341f2c1d6ed4b79c403bfb)