units: fix system.slice to require -.slice, instead of just want it

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=).

logind: add infrastructure to keep track of machines, and move to slices - This changes all logind cgroup objects to use slice objects rather than fixed croup locations. - logind can now collect minimal information about running VMs/containers. As fixed cgroup locations can no longer be used we need an entity that keeps track of machine cgroups in whatever slice they might be located. Since logind already keeps track of users, sessions and seats this is a trivial addition. - nspawn will now register with logind and pass various bits of metadata along. A new option "--slice=" has been added to place the container in a specific slice. - loginctl gained commands to list, introspect and terminate machines. - user.slice and machine.slice will now be pulled in by logind.service, since only logind.service requires this slice.

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