Prevent an infinite loop in shutdown_listener()

The loop in shutdown_listener() assumes that the reference count for every controlconnection_t object on the listener->connections linked list will drop down to zero after the conn_shutdown() call in the loop's body. However, when the timing is just right, some netmgr callbacks for a given control connection may still be awaiting processing by the same event loop that executes shutdown_listener() when the latter is run. Since these netmgr callbacks must be run in order for the reference count for the relevant controlconnection_t objects to drop to zero, when the scenario described above happens, shutdown_listener() runs into an infinite loop due to one of the controlconnection_t objects on the listener->connections linked list never going away from the head of that list.

Fix by safely iterating through the listener->connections list and initiating shutdown for all controlconnection_t objects found. This allows any pending netmgr callbacks to be run by the same event loop in due course, i.e. after shutdown_listener() returns.

Closes #4060 (closed)