libctnetlink updates

This patch reworks libctnetlink to support conntrack netlink attributes. It's required to keep in sync changes with ip_conntrack_netlink.

Several structures like: ctnl_tuple, ctnl_l4, ctnl_counters... has been introduced, this are *not* sent to kernel space, they are used to build the netlink message with nested attributes, so consider them just a container of information required to build up the netlink messages with nested attributes ;).

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Pablo