logind: enable wall messages by default

logind: enable wall messages by default

Something calling directly into the dbus interface to request a
shutdown may not bother turning wall messages on explicitly.

This has the convenient side effect that no separate polkit auth is
required to turn on wall messages. Was annoying as having a wall
message is the default behavior of the commandline tools. Now it's
the other way around ie eg systemctl reboot --no-wall requires auth
to explicitly turn off the wall message.