kernels: Add 'kernel-sockets'.

* jupyter/kernels.scm (kernel-sockets): New procedure.
(close-kernel): Use it.

jupyter/kernels.scm

@@ -40,6 +40,7 @@
             kernel-heartbeat
             kernel-iosub
             kernel-iopub                          ;alias
+            kernel-sockets
 
             read-message
             send-message
@@ -82,14 +83,19 @@
   (%kernel name pid key
            control shell standard-input heartbeat iosub))
 
+(define (kernel-sockets kernel)
+  "Return all the ZeroMQ sockets associated with KERNEL."
+  (map (lambda (socket)
+         (socket kernel))
+       (list kernel-shell
+             kernel-control
+             kernel-standard-input
+             kernel-heartbeat
+             kernel-iopub)))
+
 (define (close-kernel kernel)
   "Close all the open connections of KERNEL."
-  (for-each zmq-close-socket
-            (map (lambda (proc)
-                   (proc kernel))
-                 (list kernel-control kernel-shell
-                       kernel-standard-input
-                       kernel-heartbeat kernel-iopub))))
+  (for-each zmq-close-socket (kernel-sockets kernel)))
 
 (define (jupyter-kernel-path)
   "Return the default search path for Jupyter kernels."