bug#44320: [PATH] [27.1] Make sure send-string-to-terminal send all contents

[Eli Zaretskii]

> Date: Fri, 30 Oct 2020 07:01:53 +0800
> From: Lin Sun <lin.sun@zoom.us>
> 
> I got a bug that `send-string-to-termina` maybe send part of contents to
> terminal, calling `fwrite` in loop to make sure all contents are written.

Any idea why does it not write the entire string in the single call,
or how to reproduce this?  What is the size of the string in the case
where it fails to write everything?

Thanks.