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ipflter: ANSIfy userland function declarations

Convert ipfilter userland function declarations from K&R to ANSI. This
syncs our function declarations with NetBSD hg commit 75edcd7552a0
(apply our changes). Though not copied from NetBSD, this change was
partially inspired by NetBSD's work and inspired by style(9).

Reviewed by: glebius (for #network)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33595

ipfilter: Move userland bits to sbin

Through fixes and improvements our ipfilter sources have diverged
enough to warrant move from contrib into sbin/ipf. Now that I'm
planning on implementing MSS clamping as in iptables it makes more
sense to move ipfilter to sbin.

This is the second of three commits of the ipfilter move.

Suggested by glebius on two occaions.

Suggested by and discussed with: glebius
Reviewed by: glebius, kp (for #network)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33510