pf: add a way to list creator ids

Allow userspace to retrieve a list of distinct creator ids for the
current states.

This is used by pfSense, and used to require dumping all states to
userspace. It's rather inefficient to export a (potentially extremely
large) state table to obtain a handful (typically 2) of 32-bit integers.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42092