cgroup: don't trim cgroup trees created by someone else

cgroup: don't trim cgroup trees created by someone else

In cases when there is a cgroup tree in a controller hierarchy which was
not created by us, but it looks like it was (i.e. cgroup path is the
same as the one in systemd's named hierarchy) we shouldn't delete it.

Origin: http://lists.freedesktop.org/archives/systemd-devel/2014-September/023276.html
Bug-Debian: https://bugs.debian.org/777601

Gbp-Pq: Topic debian
Gbp-Pq: Name cgroup-don-t-trim-cgroup-trees-created-by-someone-el.patch