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.

src/shared/meson.build

@@ -251,6 +251,7 @@ shared_sources = files(
         'varlink-io.systemd.Resolve.Monitor.c',
         'varlink-io.systemd.Shutdown.c',
         'varlink-io.systemd.StorageProvider.c',
+        'varlink-io.systemd.SysUpdate.c',
         'varlink-io.systemd.SysUpdate.Notify.c',
         'varlink-io.systemd.Udev.c',
         'varlink-io.systemd.Unit.c',

src/shared/varlink-io.systemd.SysUpdate.c

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "bus-polkit.h"
+#include "varlink-io.systemd.SysUpdate.h"
+
+SD_VARLINK_DEFINE_INTERFACE(
+                io_systemd_SysUpdate,
+                "io.systemd.SysUpdate",
+                SD_VARLINK_INTERFACE_COMMENT("APIs to manage system updates"));

src/shared/varlink-io.systemd.SysUpdate.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "sd-varlink-idl.h"
+
+extern const sd_varlink_interface vl_interface_io_systemd_SysUpdate;