This commit adds the conntrack module.

It is a connection tracker that resides entirely in userspace.  Its
primary user will be the dpif-netdev datapath.

The module main goal is to provide conntrack_execute(), which offers a
convenient interface to implement the datapath ct() action.

The conntrack module uses two submodules to deal with the l4 protocol
details (conntrack-other for UDP and ICMP, conntrack-tcp for TCP).

The conntrack-tcp submodule implementation is adapted from FreeBSD's pf
subsystem, therefore it's BSD licensed.  It has been slightly altered to
match the OVS coding style and to allow the pickup of already
established connections.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Antonio Fischetti <antonio.fischetti@intel.com>
Acked-by: Joe Stringer <joe@ovn.org>