bug#12972: [PATCH] Avoid regression in mailcap-view-file similar to Bug#44824

[Maxim Nikulin]

By mistake I sent the message below as private one at first. However it 
actually does not add anything new to my previous comments to the bug.

On 04/07/2021 20:49, Eli Zaretskii wrote:
> > From: Maxim Nikulin Date: Sun, 4 Jul 2021 20:37:24 +0700
>
> Sorry, I'm not sure I understand what this is all about.  Are you
> still talking about the patch you proposed?

Yes, I am. It is about proper way to a launch viewer in 
`mailcap-view-file'. Original `start-process-shell-command' with 'pty 
connection type prematurely kills children of kde-open5 or gio open. 
With 'pipe connection type it or `make-process' might make emacs CPU 
hungry if a child decides to close stdout and stderr:

> > #!/bin/sh
> > exec 1>&-
> > exec 2>&-
> > sleep 30

and finally `process-file-shell-command' does not allow to report 
failure. Moreover you suspect another secret compatibility problem with 
'pipe.