systemctl: add new option to mount image inside a running service namespace

Use the new DBUS method and follow the same pattern as the
systemctl bind command.

src/systemctl/systemctl-mount.h

@@ -2,3 +2,4 @@
 #pragma once
 
 int mount_bind(int argc, char *argv[], void *userdata);
+int mount_image(int argc, char *argv[], void *userdata);