pf: match keyword support

Support the 'match' keyword.
Note that support is limited to adding queuing information, so without
ALTQ support in the kernel setting match rules is pointless.

For the avoidance of doubt: this is NOT full support for the match
keyword as found in OpenBSD's pf. That could potentially be built on top
of this, but this commit is NOT that.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31115