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allows checking on boot whether /etc or /var are outdated and need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.

Makefile.am

@@ -361,7 +361,8 @@ rootlibexec_PROGRAMS = \
 	systemd-sysctl \
 	systemd-sleep \
 	systemd-bus-proxyd \
-	systemd-socket-proxyd
+	systemd-socket-proxyd \
+	systemd-update-done
 
 systemgenerator_PROGRAMS = \
 	systemd-getty-generator \
@@ -495,7 +496,8 @@ nodist_systemunit_DATA = \
 	units/initrd-cleanup.service \
 	units/initrd-udevadm-cleanup-db.service \
 	units/initrd-switch-root.service \
-	units/systemd-nspawn@.service
+	units/systemd-nspawn@.service \
+	units/systemd-update-done.service
 
 dist_userunit_DATA = \
 	units/user/basic.target \
@@ -538,7 +540,8 @@ EXTRA_DIST += \
 	units/initrd-cleanup.service.in \
 	units/initrd-udevadm-cleanup-db.service.in \
 	units/initrd-switch-root.service.in \
-	units/systemd-nspawn@.service.in
+	units/systemd-nspawn@.service.in \
+	units/systemd-update-done.service.in
 
 CLEANFILES += \
 	units/console-shell.service.m4 \
@@ -1640,6 +1643,14 @@ systemd_update_utmp_LDADD = \
 	libsystemd-shared.la \
 	$(AUDIT_LIBS)
 
+# ------------------------------------------------------------------------------
+systemd_update_done_SOURCES = \
+	src/update-done/update-done.c
+
+systemd_update_done_LDADD = \
+	libsystemd-internal.la \
+	libsystemd-shared.la
+
 # ------------------------------------------------------------------------------
 systemd_shutdownd_SOURCES = \
 	src/shutdownd/shutdownd.c
@@ -5100,14 +5111,19 @@ RUNLEVEL4_TARGET_WANTS += \
 RUNLEVEL5_TARGET_WANTS += \
 	systemd-update-utmp-runlevel.service
 endif
+
 SYSINIT_TARGET_WANTS += \
-	systemd-update-utmp.service
+	systemd-update-utmp.service \
+	systemd-update-done.service
+
 LOCAL_FS_TARGET_WANTS += \
 	systemd-remount-fs.service \
 	tmp.mount
+
 MULTI_USER_TARGET_WANTS += \
 	getty.target \
 	systemd-ask-password-wall.path
+
 SYSINIT_TARGET_WANTS += \
 	dev-hugepages.mount \
 	dev-mqueue.mount \