pf: Move nvlist conversion functions to pf_nv

Separate the conversion functions (between kernel structs and nvlists)
to pf_nv. This reduces the size of pf_ioctl.c, which is already quite
large and complex, a good bit. It also keeps all the fairly
straightforward conversion code together.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30359

(cherry picked from commit 3032c353882aa70a461ef251b8aa6c59a0829ec5)