cgroup.c: redefine the valid cgroup name

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
cgroup.h: unify the nameing and comments

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
cgroup.c: add static keywords as they declared

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>