Features:
+* add "requires=" deps on slices from services, not just "wants="
+
+* add a concept of RemainAfterExit= to scope units
+
+* add sd_booted() check into "systemctl is-system-running", and return
+ a new state "foreign" or so if we are not running on systemd.
+
+* add journal vacuum by max number of files
+
+* add a new command "systemctl revert" or so, that removes all dropin
+ snippets in /run and /etc, and all unit files with counterparts in
+ /usr, and thus undoes what "systemctl set-property" and "systemctl
+ edit" create. Maybe even add "systemctl revert -a" to do this for
+ all units.
+
* sd-event: maybe add support for inotify events
* PID 1 should send out sd_notify("WATCHDOG=1") messages (for usage in the --user mode, and when run via nspawn)