update-done: add minimal tool to manage system updates for /etc and /var, if /usr has changed

In order to support offline updates to /usr, we need to be able to run
certain tasks on next boot-up to bring /etc and /var in line with the
updated /usr. Hence, let's devise a mechanism how we can detect whether
/etc or /var are not up-to-date with /usr anymore: we keep "touch
files" in /etc/.updated and /var/.updated that are mtime-compared with
/usr. This means:

Whenever the vendor OS tree in /usr is updated, and any services that
shall be executed at next boot shall be triggered, it is sufficient to
update the mtime of /usr itself. At next boot, if /etc/.updated and/or
/var/.updated is older than than /usr (or missing), we know we have to
run the update tools once. After that is completed we need to update the
mtime of these files to the one of /usr, to keep track that we made the
necessary updates, and won't repeat them on next reboot.

A subsequent commit adds a new ConditionNeedsUpdate= condition that
allows checking on boot whether /etc or /var are outdated and need
updating.

This is an early step to allow booting up with an empty /etc, with
automatic rebuilding of the necessary cache files or user databases
therein, as well as supporting later updates of /usr that then propagate
to /etc and /var again.

units/.gitignore

@@ -65,6 +65,7 @@
 /systemd-udevd.service
 /systemd-update-utmp-runlevel.service
 /systemd-update-utmp.service
+/systemd-update-done.service
 /systemd-user-sessions.service
 /systemd-vconsole-setup.service
 /user@.service

units/systemd-update-done.service.in

@@ -0,0 +1,21 @@
+#  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=Update is Completed
+Documentation=man:sysusers.d(5) man:systemd-sysusers(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+After=systemd-readahead-collect.service systemd-readahead-replay.service local-fs.target
+Before=sysinit.target shutdown.target
+RefuseManualStart=yes
+RefuseManualStop=yes
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=@rootlibexecdir@/systemd-update-done