scm: support --listen flag for uds

Support the --listen flag to mu scm, to start listening on a Unix domain socket.

scm/mu-scm-repl.scm

@@ -0,0 +1,28 @@
+;; Copyright (C) 2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+;;
+;; 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 the
+;; Free Software Foundation; either version 3, or (at your option) any
+;; later version.
+;;
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with this program; if not, write to the Free Software Foundation,
+;; Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+(use-modules (system repl server)
+	     (ice-9 threads))
+(use-modules (mu))
+
+;; when a socket path is defined, listen on it (blocking)
+;; after printing UNIX-CONNECT:<socket-file>\n on stdout
+(let ((socket-path (getenv "MU_SCM_SOCKET_PATH")))
+  (when socket-path
+    (format #t "UNIX-CONNECT:~a\n" socket-path)
+    (run-server
+     (make-unix-domain-server-socket #:path socket-path))))
+
+(display "Welcome to the mu shell!\n\n")