wall(1)-like system notifications

It would be great to have wall(1)-like functionality supported by libnotify. Something like a notify-send-all, that gets the list of interactive sessions from systemd logind and then sends a notification to each of those session (as if notify-send was called for each).

For use-cases related to the earlyoom system daemon see:

• https://github.com/rfjakob/earlyoom/pull/182
• https://github.com/rfjakob/earlyoom/issues/183
• https://github.com/rfjakob/systembus-notify

But I assume there will be lots of use-cases involving other system daemons.