Add support for making the supervisor listen for requests

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.

scripts/base/frameworks/supervisor/control.zeek

@@ -4,6 +4,7 @@
 ##! That is, it may change in various incompatible ways without warning or
 ##! deprecation until the stable 4.0.0 release.
 
+@load base/frameworks/broker
 @load ./api
 
 module SupervisorControl;
@@ -15,6 +16,10 @@ export {
 	## for their topic names.
 	const topic_prefix = "zeek/supervisor" &redef;
 
+	## When enabled, the Supervisor will listen on the configured Broker
+	## :zeek:see:`Broker::default_listen_address`.
+	const enable_listen = F &redef;
+
 	## Send a request to a remote Supervisor process to create a node.
 	##
 	## reqid: an arbitrary string that will be directly echoed in the response

scripts/base/frameworks/supervisor/main.zeek

@@ -3,7 +3,6 @@
 
 @load ./api
 @load ./control
-@load base/frameworks/broker
 
 function Supervisor::status(node: string): Supervisor::Status
 	{
@@ -42,6 +41,13 @@ function Supervisor::node(): Supervisor::NodeConfig
 
 event zeek_init() &priority=10
 	{
+	if ( Supervisor::is_supervisor() && SupervisorControl::enable_listen )
+		{
+		Broker::listen(Broker::default_listen_address,
+			       Broker::default_port,
+			       Broker::default_listen_retry);
+		}
+
 	Broker::subscribe(SupervisorControl::topic_prefix);
 	}