src: add list ruleset command

src: add list ruleset command

This patch adds a new command to nft:
	% nft list ruleset [family]

Which list the entire ruleset.
If no family is specified, all tables of all families are listed.

Users can now make several operations at ruleset level:

 % nft list ruleset > ruleset.nft
 % nft -f ruleset.nft
 % nft flush ruleset

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