CGroup Rework Completion:
-* systemctl user.slice doesn't show the full tree?
-
* introduce "mainpid" for scopes
* implement system-wide DefaultCPUAccounting=1 switch (and similar for blockio, memory?)
* logind: implement session kill exceptions
-* fix machine regstration to forward property array
-
* make BlockIODeviceWeight=, BlockIODeviceBandwidth= runtime settable
-* split up BlockIOWeight= and BlockIODeviceWeight=
-
* introduce high-level settings for RT budget, swappiness
* man: document new bus apis