logind: add PrepareForShutdownWithMetadata signal

logind: add PrepareForShutdownWithMetadata signal

The existing signal doesn't say which type of shutdown is going to happen.
With the introduction of soft-reboot, it is useful to have this information
broadcasted, so that clients can choose to do different things based on the
reboot type.
Add a{sv} as the payload so that more metadata can be added later if
needed, without needing to add yet another signal.
Send both old and new signal for backward compatibility, and send the new
one first so that clients can just wait for the first one on both old and
new systems.