logind/systemctl: introduce SetWallMessage and --message

logind/systemctl: introduce SetWallMessage and --message

Enable unprivileged users to set wall message on a shutdown
operation. When the message is set via the --message option,
it is logged together with the default shutdown message.

$ systemctl reboot --message "Applied kernel updates."

$ journalctl -b -1
...
systemd-logind[27]: System is rebooting. (Applied kernel updates.)
...