The supervisor now starts listening on the configured Broker default address and
port when the new boolean SupervisorControl::enable_listen is T. Listening
remains disabled by default. Listening allows nodes to communicate with the
supervisor via the events laid out in control.zeek, to conduct further node
management.
Also have stem process execv() with original command-line arguments so
that they're re-parsed and inherited correctly by supervised-nodes in
the event the stem process needs to be re-created.
More aspects of the cluster configuration to get fleshed out later,
but a basic cluster like one would use for a live deployment
can now be instantiated and run under supervision. The new
clusterized-pcap-processing supervisor mode is also not done yet.
