pf: Optionally attempt to preserve rule counter values across ruleset updates

Usually rule counters are reset to zero on every update of the ruleset.
With keepcounters set pf will attempt to find matching rules between old
and new rulesets and preserve the rule counters.

MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29780

(cherry picked from commit 42ec75f83aa321fcea8d2eddc4e9099724f0669e)