This adds the scaffolding for being able to call sysupdate via varlink,
but it doesn’t yet define or implement any methods. Those will come in
following commits.
The existing `systemd-sysupdate.service` and `systemd-sysupdate.timer`
(which periodically ran `systemd-sysupdate update`) have been renamed
to `systemd-sysupdate-update.{service,timer}` to make way for a new
`systemd-sysupdate@.service` and `systemd-sysupdate.socket` file to
handle varlink activation.
Compatibility symlinks have been added for them.
