`SystemdCgroup` takes a `parent`, which is the name of a slice, and a
`unit`, which is the name of a slice or a scope unit, and provides methods
to start, kill the unit, as well as set properties for the unit.
The mods, `cpu`, `memory`, `cpuset`, and `pids`, are designed to generate
properties quickly. It hides the difference between cgroups v1 and v2, and
does simple checks for the systemd version and arguments.
Signed-off-by: Xuewei Niu <niuxuewei.nxw@antgroup.com>
