Try to block system shutdown when updater is running

We should block system shutdown when updater is running to make chance of breakage because of reboot in inappropriate moment minimal.

On ProcD it seems that we can do it by just writing stop service that blocks shutdown till update terminates.

Problem is that at the moment when updater is running reboot as part of its execution it infinitely blocks instead of exit. We can't easily recognize this state and so we can just simply check for updater process.