Prep v231.2: Fix missing/double wall msgs on scheduled shutdown/reboot

Prep v231.2: Fix missing/double wall msgs on scheduled shutdown/reboot

- Do not do anything in manager_setup_wall_message_timer() if wall
  messages are disabled anyway.
- Set up a wall timer in any case there is time left. The original
  sources would not even set up a timer if the next messages would
  be now. As time is measured in USEC, that's pretty rare, but
  possible.
- If less than 1 Second is left to the first message, delay it.
- systemd would print out a message at once, if less than 15 minutes
  are left to the event. Do this only, if the next scheduled message
  wouldn't come within the next 3 seconds, or it might come to
  awkward double messages.