kernel: Log startup.

* guix-jupyter-kernel.scm <top level>: Log kernel startup.

guix-jupyter-kernel.scm

 ;;; Guix-kernel -- Guix kernel for Jupyter
 ;;; Copyright (C) 2018 Evgeny Panfilov <epanfilov@gmail.com>
 ;;; Copyright (C) 2018 Pierre-Antoine Rouby <pierre-antoine.rouby@inria.fr>
-;;; Copyright (C) 2018, 2019, 2020 Inria
+;;; Copyright (C) 2018, 2019, 2020, 2021 Inria
 ;;;
 ;;; This program is free software: you can redistribute it and/or modify
 ;;; it under the terms of the GNU General Public License as published by
@@ -597,6 +597,7 @@ environment ~s (PID ~s)~%"
   (sigaction SIGTERM (exit-handler kernel))
   (sigaction SIGINT  (exit-handler kernel))
 
+  (format/log "Guix kernel started (PID ~a)~%" (getpid))
   (with-store store
     (serve-kernels (list kernel)
                    (proxy-request-handler dispatch-route)