scm: implement blocking / non-blocking modes

Implement running the REPL on background thread. That way, we can _share_ the
store&.

scm/mu-scm-repl.scm

@@ -21,7 +21,7 @@
 ;; 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)
+    (format #t "~a\n" socket-path)
     (run-server
      (make-unix-domain-server-socket #:path socket-path))))