Merge pull request #8149 from poettering/fake-root-cgroup

Properly synthesize CPU+memory accounting data for the root cgroup

diff --cc TODO
Simple merge

diff --cc src/basic/cgroup-util.c
Simple merge

diff --cc src/cgtop/cgtop.c
Simple merge

diff --cc src/core/cgroup.c
Simple merge

diff --cc src/core/cgroup.h
Simple merge

diff --cc src/core/slice.c
Simple merge