[m-rev.] for review: Improve wait_pid and wait_any.

[Julien Fischer]

Hi Peter,

On Thu, 2 Jul 2020, Peter Wang wrote:

> compiler/process_util.m:
>    Make do_wait always assign all output variables.
>
>    Make do_wait return an error message if waitpid() fails.
>    Do not use MR_perror.
>
>    Make wrappers wait_pid and wait_any return errors from waitpid().
>
>    Change wait_any to return maybe(pid) in case waitpid() does not
>    return a child process id.

That's fine.

Julien.