This is a followup to the following emacs-devel thread:
https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg00880.html

The user option explicit-shell-file-name has been defined in three
different places and under three different custom groups since at least
as far back as 1997:

- lisp/shell.el
- lisp/term.el
- lisp/obsolete/terminal.el

I think there should be only a single definition, and lisp/shell.el was
suggested as the correct place in the aforementioned thread.

I attach a patch which does this and also cleans up a couple of uses of
explicit-shell-file-name.  Tino, was there a particular reason for using
explicit-shell-file-name instead of shell-file-name in
dired-insert-directory[1], or is it safe to swap them?

[1]: Don't assume /bin/sh as the 'sh' location in the local host
e82c4f56e6 2017-08-02 16:50:44 +0900
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=e82c4f56e6f9a6bce4098698b17fa45dcc5bbd25

Thanks,

-- 
Basil