pf: Introduce ridentifier

Allow users to set a number on rules which will be exposed as part of
the pflog header.
The intent behind this is to allow users to correlate rules across
updates (remember that pf rules continue to exist and match existing
states, even if they're removed from the active ruleset) and pflog.

Obtained from: pfSense
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32750