src: add support for stateful object maps

src: add support for stateful object maps

You can create these maps using explicit map declarations:

 # nft add table filter
 # nft add chain filter input { type filter hook input priority 0\; }
 # nft add map filter badguys { type ipv4_addr : counter \; }
 # nft add rule filter input counter name ip saddr map @badguys
 # nft add counter filter badguy1
 # nft add counter filter badguy2
 # nft add element filter badguys { 192.168.2.3 : "badguy1" }
 # nft add element filter badguys { 192.168.2.4 : "badguy2" }

Or through implicit map definitions:

 table ip filter {
        counter http-traffic {
                packets 8 bytes 672
        }

        chain input {
                type filter hook input priority 0; policy accept;
                counter name tcp dport map { 80 : "http-traffic", 443 : "http-traffic"}
        }
 }

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