[RFC] Consider adding a function to convert error name to errno

[Rocket Ma]

Got it. Then what about return the errno back, instead of writing it
into a pointer? I have some rationales:

1. The old interfaces like strerror directly return string, instead of
writing error desc into a pointer, so directly returning the errno
preserves the "convention".
2. Error names are fixed, while sigal names are not fixed, for
example, SIGRTMIN+1.
3. Directly return the errno reduce some code complexity.
4. Errnos are all positive, so returning -1 could indicate error.

In this case, we have the function signature:

  int strnameerror_np(const char *errname);

> * Rocket Ma:
>
> The prefix “str” makes the name reserved.  We have function like strtod
> that produce non-string results.
>
> Thanks,
> Florian
>