connmark: Add CONNMARK rules to select correct output SA based on conntrack

Currently supports transport mode connections using IPv4 only, and requires
a unique mark configured on the connection.

To select the correct outbound SA when multiple connections match (i.e.
multiple peers connected from the same IP address / NAT router) marks must be
configured. This mark should usually be unique, which can be configured in
ipsec.conf using mark=0xffffffff.

The plugin inserts CONNMARK netfilter target rules: Any peer-initiated flow
is tagged with the assigned mark as connmark. On the return path, the mark
gets restored from the conntrack entry to select the correct outbound SA.
ipsec-types: Support the %unique mark value
travis: Install iptables-dev for connmark plugin in "all" tests
testing: Install iptables-dev to guest images
testing: Build strongSwan with the connmark plugin
testing: Add a connmark plugin test

In this test two hosts establish a transport mode connection from behind
moon. sun uses the connmark plugin to distinguish the flows.

This is an example that shows how one can terminate L2TP/IPsec connections
from two hosts behind the same NAT. For simplification of the test, we use
an SSH connection instead, but this works for any connection initiated flow
that conntrack can track.
NEWS: Introduce connmark plugin