sysext: refresh sysexts and confexts on completed system update

Bind the io.systemd.SysUpdate.Notify.OnCompletedUpdate() method in the
sysext Varlink server. systemd-sysext provides a single Varlink service
covering both the sysext and confext image classes, so one notification
refreshes both (equivalent to "systemd-sysext refresh" plus
"systemd-confext refresh"). Hook a socket into
/run/systemd/sysupdate/notify/ via systemd-sysupdate-notify-sysext.socket,
enabled by default via the preset.

units/meson.build

@@ -823,6 +823,14 @@ units = [
           'conditions' : ['ENABLE_SYSEXT'],
           'symlinks' : ['sockets.target.wants/'],
         },
+        {
+          'file' : 'systemd-sysupdate-notify-sysext@.service',
+          'conditions' : ['ENABLE_SYSEXT'],
+        },
+        {
+          'file' : 'systemd-sysupdate-notify-sysext.socket',
+          'conditions' : ['ENABLE_SYSEXT'],
+        },
         {
           'file' : 'systemd-sysext@.service',
           'conditions' : ['ENABLE_SYSEXT'],

units/systemd-sysupdate-notify-sysext.socket

@@ -0,0 +1,29 @@
+#  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 Extension and Configuration Refresh on Completed System Update
+Documentation=man:systemd-sysext(8)
+DefaultDependencies=no
+After=local-fs.target
+Before=sockets.target
+ConditionCapability=CAP_SYS_ADMIN
+
+[Socket]
+ListenStream=/run/systemd/sysupdate/notify/io.systemd.sysext
+FileDescriptorName=varlink
+SocketMode=0644
+Accept=yes
+MaxConnectionsPerSource=16
+XAttrEntryPoint=user.varlink=entrypoint
+XAttrListen=user.varlink=listen
+XAttrAccept=user.varlink=server
+
+[Install]
+WantedBy=sockets.target

units/systemd-sysupdate-notify-sysext@.service

@@ -0,0 +1,19 @@
+#  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 Extension and Configuration Refresh on Completed System Update
+Documentation=man:systemd-sysext(8)
+DefaultDependencies=no
+Conflicts=shutdown.target initrd-switch-root.target
+After=local-fs.target
+Before=shutdown.target initrd-switch-root.target
+
+[Service]
+ExecStart=-systemd-sysext