sysupdate: Add basic varlink interface scaffolding

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.

units/systemd-sysupdate.socket

@@ -0,0 +1,28 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=System Updates
+Documentation=man:systemd-sysupdate(8)
+DefaultDependencies=no
+Before=sockets.target
+
+[Socket]
+ListenStream=/run/systemd/io.systemd.SysUpdate
+Symlinks=/run/varlink/registry/io.systemd.SysUpdate
+FileDescriptorName=varlink
+SocketMode=0666
+Accept=yes
+MaxConnectionsPerSource=16
+XAttrEntryPoint=user.varlink=entrypoint
+XAttrListen=user.varlink=listen
+XAttrAccept=user.varlink=server
+
+[Install]
+WantedBy=sockets.target