pf: fallback if $pf_rules fails to load

Support loading a default pf ruleset in case of invalid pf.conf.

If no pf rules are loaded pf will pass/allow all traffic, assuming the
kernel is compiled without PF_DEFAULT_TO_DROP, as is the case in
GENERIC.

In other words: if there's a typo in the main pf_rules we would allow
all traffic. The new default rules minimise the impact of this.

If $pf_program (i.e. pfctl) fails to set $pf_fules and
$pf_fallback_rules_enable is YES we will load $pf_fallback_rules_file if
set, or $pf_fallback_rules.

$pf_fallback_rules can include multiple rules, for example to permit
traffic on a management interface.

$pf_fallback_rules_enable defaults to "NO", preserving historic behaviour.

man page changes by ceri@.

PR: 256410
Reviewed by: donner, kp
Sponsored by: semaphor.dk
Differential Revision: https://reviews.freebsd.org/D30791