[script splash] 'plymouth message' before 'plymouth show-splash' doesn't render

Binary package hint: plymouth

If 'plymouth message' is called before 'plymouth show-splash' (e.g., by mountall), the script callback will never actually be called,
potentially leaving the user with no explanation for the delay nor the hints about keypresses.

The message callback still needs to be called somehow (queued in plymouthd?) even if the splash screen isn't yet up when the request comes in.