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