pf: add a dedicated pf pool for route options

As suggested by henning.
Which unbreaks ie route-to after the recent pf changes.

With much help debugging and pointing out of missing bits from claudio@

ok claudio@ "looks good" henning@

Obtained from: OpenBSD, jsg <jsg@openbsd.org>, 7fa5c09028
Sponsored by: Rubicon Communications, LLC ("Netgate")