libpfctl: migrate to DIOCGETSTATESV2

Stop using the *NV version to retrieve states, as its performance is
unacceptably bad.

For 1,000,000 states the nvlist version needed ~100 seconds to retrieve
the states, the new version needs ~3 seconds.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31098

(cherry picked from commit be70c7a50d324dd56f3a0d37d8372e7855dd580b)