pf: Migrate pf_rule and related structs to pf.h

As part of the split between user and kernel mode structures we're
moving all user space usable definitions into pf.h.

No functional change intended.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27757

(cherry picked from commit dc865dae89c34291467e0ba569a8c78fc7a5d360)