src: support for counter in set definition

src: support for counter in set definition

This patch allows you to turn on counter for each element in the set.

 table ip x {
	set y {
		typeof ip saddr
		counter
		elements = { 192.168.10.35, 192.168.10.101, 192.168.10.135 }
	}

	chain z {
		type filter hook output priority filter; policy accept;
		ip daddr @y
	}
 }

This example shows how to turn on counters globally in the set 'y'.

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