bug#54062: 29.0.50; [PATCH] Eshell should inform processes when a pipe is broken

[Eli Zaretskii]

> From: Jim Porter <jporterbugs@gmail.com>
> Date: Fri, 18 Feb 2022 20:20:10 -0800
> 
> Consider the following shell command:
> 
>    yes | sh -c 'read NAME'
> 
> Ordinarily, you'd expect that `sh' reads a single "y", exits, and then 
> the next time `yes' tries to write, it finds that the pipe was broken. 
> However, that's not what happens in Eshell. Running the above and then 
> calling `M-x list-processes' will show that `yes' is still running.
> 
> Attached is a patch (with a test) to fix this by telling Eshell to 
> signal SIGPIPE at the appropriate time.

SIGPIPE isn't supported on MS-Windows, so I think we should have a
fallback there for platforms that don't support SIGPIPE.

Thanks.