jack_control: new command - "show"

show command behaves a bit like last parts of (current) adevices.sh script,
but chains the involved commands status2, dg, dp and ep internally.

The adevices.sh script uses status command and does not display
JACK settings unless jack is started. The show command uses
status2, so the settings will be displayed always.

dbus/jack_control

@@ -162,6 +162,8 @@ def parse_argv(argv):
                     print("started")
                 else:
                     print("stopped")
+            elif arg == "show":
+                parse_argv(["status2", "dg", "dp", "ep"])
             elif arg == 'start':
                 print("--- start")
                 control_iface.StartServer()