netlink-conntrack: New module.

netlink-conntrack: New module.

This module uses the netlink interface provide by the Linux kernel
connection tracker to provide some visibility into the conntrack tables.

The module provides functions to:

* Convert a netlink representation of a connection into a
  struct 'ct_dpif_entry'.

* Dump all the connections.

* Flush all the connections.

* Listen for updates by registering a netlink notifier.

It will be used by dpif-netlink to implement the interface required by
the ct-dpif module.

Based on original work by Jarno Rajahalme

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>