Generalize the libct/getValueFromCgroup() as fscommon.GetValueByKey(),
and document it.
No changes other than using fscommon.ParseUint to convert the value.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This is to benefit from openat2() implementation, on kernels
that support it. Theoretically this also improves security.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>