Features:
+* introduce bus call FreezeUnit(s, b), as well as "systemctl freeze
+ $UNIT" and "systemctl thaw $UNIT" as wrappers around this. The calls
+ should SIGSTOP all unit processes in a loop until all processes of
+ it are fully stopped. This can later be used for app management by
+ desktop UIs such as gnome-shell to freeze apps that are not visible
+ on screen, not unlike how job control works on the shell
+
* machinectl bash completion is missing
* busctl bash and zsh completions are outdated