cgroup: SingleControlGroup implementation + detail::{Attached,Created}ControlGroup stubs.

SingleControlGroup: root(), attachDirectChild() and createDirectChild() implementation.
cgroup::SingleControlGroup::attach() bug fix.
cgroup::SingleControlGroup: {attach,create}{,Direct}Child() bug fixes.
cgroup test suite: improvements + SingleControlGroup::tree test case implementation.
cgroup::SingleControlGroup::mountpoint() implementation.
cgroup::SingleControlGroup::SingleControlGroup() additional assert.