src: add `list chains' command

src: add `list chains' command

 # nft list chains
 table ip filter {
        chain test1 {
        }
        chain test2 {
        }
        chain input {
                type filter hook input priority 0; policy accept;
        }
 }
 table ip6 filter {
        chain test1 {
        }
        chain input {
                type filter hook input priority 0; policy accept;
        }
 }

You can also filter out per family:

 # nft list chains ip
 table ip x {
        chain y {
        }
        chain xz {
        }
        chain input {
                type filter hook input priority 0; policy accept;
        }
 }

 # nft list chains ip6
 table ip6 filter {
        chain x {
        }
        chain input {
                type filter hook input priority 0; policy accept;
        }
 }

This command only shows the chain declarations, so the content (the
definition) is omitted.

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