Fix error of parser:
> error get info about worker process status 3150292: strconv.ParseInt:
> parsing "4294967293": value out of range
$ grep Groups /proc/self/status
Groups: 20001 [...] 4294967293
Removes need for redundant ParentWithContext implementations. It had led
to it being unsupported on FreeBSD and OpenBSD even though
PpidWithContext was available for them, and different implementations
for getting the parent info used in ParentWithContext and
PpidWithContext on Darwin and Linux.
