Server Return mode, where not all packets would be visible to the load
balancer or gateway.
This commit should be reverted when we merge future pf versions. The
benefit it would provide is that this version does not break any existing
public interface and thus won't be a problem if we want to MFC it to
earlier FreeBSD releases.
Discussed with: mlaier
Obtained from: OpenBSD
Sponsored by: iXsystems, Inc.
MFC after: 1 month
dcc2b1ff46