rule: allow to print sets in plain format

rule: allow to print sets in plain format

Allow to print sets with or without format.

This is useful in situations where we want to print more or less the same
the user typed (IOW, in one single line, and with family/table info).

While at it, make family2str() function public, so it can be used in
other places.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>