scm: support --listen flag for uds

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

mu/mu-options.cc

@@ -632,8 +632,11 @@ sub_server(CLI::App& sub, Options& opts)
 static void
 sub_scm(CLI::App& sub, Options& opts)
 {
+	sub.add_flag("--listen", opts.scm.listen,
+		       "Start listening on a domain socket");
 	sub.add_option("script-path", opts.scm.script_path, "Path to script")
-		->type_name("<path>");
+		->type_name("<path>")
+		->excludes("--listen");
 	sub.add_option("script-args", opts.scm.params, "Parameters for script")
 		->type_name("<parameters>");
 }

mu/mu-options.hh

@@ -259,6 +259,7 @@ struct Options {
 	struct Scm {
 		OptString script_path; /**< Path to script (optional) */
 		StringVec params;      /**< Parameters for script (after "--") */
+		bool listen; /**< Whether to start listening on a socket */
 	} scm;