core: general cgroup rework Replace the very generic cgroup hookup with a much simpler one. With this change only the high-level cgroup settings remain, the ability to set arbitrary cgroup attributes is removed, so is support for adding units to arbitrary cgroup controllers or setting arbitrary paths for them (especially paths that are different for the various controllers). This also introduces a new -.slice root slice, that is the parent of system.slice and friends. This enables easy admin configuration of root-level cgrouo properties. This replaces DeviceDeny= by DevicePolicy=, and implicitly adds in /dev/null, /dev/zero and friends if DeviceAllow= is used (unless this is turned off by DevicePolicy=).

units: add default units for system.slice, user.slice, machine.slice