Introduce the OpenBSD syntax of "scrub" option for "match" and "pass"
rules and the "set reassemble" flag. The patch is backward-compatible,
pf.conf can be still written in FreeBSD-style.
Obtained from: OpenBSD
MFC after: never
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D38025