fw4: add support for zone log_limit

fw4: add support for zone log_limit

It is equivalent to the fw3 feature, affecting not accepted packets
and rules explicitily setting the log property.

Input rules not associated with a zone will not have log_limit.
Forward rules will use src zone log_limit or, if missing, dest zone
log_limit.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
[properly handle null zone references, whitespace and indentation cleanup,
 testcase cleanup, slight code simplification, use dot for named limit,
 properly format limit expressions]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>