pf: Reduce diffs against OpenBSD in pf_if.c

Use memset instead of bzero, memcpy instead of bcopy, and add some
optional {}s.

Reviewed by: kp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36267

(cherry picked from commit 6b598e263bebaf6664b64f80f029a6ebcfc2848d)