pf: Move prototypes for userspace functions to userspace header

These functions no longer exist in the kernel, so there's no reason to
keep the prototypes in a kernel header. Move them to pfctl where they're
actually implemented.

Reviewed by: glebius
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29643

(cherry picked from commit a9b338b260be39cc74dddb599d6c95b8794ca98b)