src: listing of stateful objects

src: listing of stateful objects

This patch allows you to dump existing stateful objects, eg.

 # nft list ruleset
 table ip filter {
        counter test {
                packets 64 bytes 1268
        }

        quota test {
                over 1 mbytes used 1268 bytes
        }

        chain input {
                type filter hook input priority 0; policy accept;
                quota name test drop
                counter name test
        }
 }

 # nft list quotas
 table ip filter {
        quota test {
                over 1 mbytes used 1268 bytes
        }
 }
 # nft list counters
 table ip filter {
        counter test {
                packets 64 bytes 1268
        }
 }

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>