mem-pool: Pass the remote IKE address, to re-acquire() an address during reauth

With make-before-break IKEv2 re-authentication, virtual IP addresses must be
assigned overlapping to the same peer. With the remote IKE address, the backend
can detect re-authentication attempts by comparing the remote host address and
port. This allows proper reassignment of the virtual IP if it is re-requested.

This change removes the mem-pool.reassign_online option, as it is obsolete now.
IPs get automatically reassigned if a peer re-requests the same address, and
additionally connects from the same address and port.
testing: Add a test scenario for make-before-break reauth without a virtual IP
testing: Add a test scenario for make-before-break reauth using a virtual IP
testing: Be a little more flexible in testing for established CHILD_SA modes

As we now print the reqid parameter in the CHILD_SA details, adapt the grep
to still match the CHILD_SA mode and protocol.
testing: Update description and test evaluation of host2host-transport-nat

As we now reuse the reqid for identical SAs, the behavior changes for
transport connections to multiple peers behind the same NAT. Instead of
rejecting the SA, we now have two valid SAs active. For the reverse path,
however, sun sends traffic always over the newer SA, resembling the behavior
before we introduced explicit SA conflicts for different reqids.
connmark: Add a plugin stub
bus: Add an ike_update() hook invoked when peer endpoints change
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
forecast: Add the broadcast/multicast forwarding plugin called forecast
forecast: Document strongswan.conf options
NEWS: Introduce forecast multicast/broadcast forwarding plugin