bug#75308: 30.0.93; man.el assume shell is bash

[Stefan Kangas <stefankangas@gmail.com>]

Ted Phelps <phelps@gnusto.com> writes:

> Steps to reproduce:
> * env SHELL=/bin/tcsh emacs -Q
> * invoke M-x man
> * request a man page (e.g. emacs)
>
> This will produce output like the following instead of the man page:
>
>     Unmatched '''.
>     BEGIN: Command not found.
>     anonblank=0: Command not found.
>     }: Command not found.
>     Illegal variable name.
>     {: Command not found.
>     }: Command not found.
>     Illegal variable name.
>     Badly placed ()'s.
>     Unmatched '''.
>
>     process exited abnormally with code 1
>
> Replacing /bin/tcsh with /bin/bash in the above command produces the
> expected man page.
>
> It looks like the behavior was broken by the following commit:
>
>     commit 861ac933dd8aed1028edc4b9142400e3702874d5
>     Author: Michael Albinus <michael.albinus@gmx.de>
>     Date:   Wed Nov 1 16:54:31 2023 +0100
>
> Many thanks,
> -Ted

Thanks for the bug report.  For emacs-30, we should look into a some
small workaround.  I'm copying in Michae.

In Emacs 31, I want to investigate not calling out to a shell at all.
The code that does that dates back to 1991... and what was a worthwhile
optimization back then is not necessarily relevant today.