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.
starter: Support the %unique mark value in ipsec.conf
travis: Install iptables-dev for connmark plugin in "all" tests