pf: create a kif for flags

If userspace tries to set flags (e.g. 'set skip on <ifspec>') and <ifspec>
doesn't exist we should create a kif so that we apply the flags when the
<ifspec> does turn up.

Otherwise we'd end up in surprising situations where the rules say the
interface should be skipped, but it's not until the rules get re-applied.

Reviewed by: Lutz Donnerhacke <lutz_donnerhacke.de>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26742