Define explicit-shell-file-name only in shell.el

For discussion, see the following thread:
https://lists.gnu.org/r/emacs-devel/2020-04/msg00880.html

* doc/emacs/cmdargs.texi (General Variables): Mention
shell-file-name in relation to SHELL.
* doc/emacs/misc.texi (Interactive Shell): Move index entry for
SHELL environment variable from here, where it is not mentioned...
(Single Shell): ...to here, where it is discussed along with
shell-file-name.
* lisp/dired.el (dired-insert-directory): Use shell-file-name
instead of explicit-shell-file-name when a shell is implicitly
requested.
* lisp/obsolete/terminal.el (explicit-shell-file-name):
* lisp/term.el (explicit-shell-file-name): Remove duplicate
defcustoms and load lisp/shell.el instead.  (Bug#40679)
* lisp/shell.el (explicit-shell-file-name): Clarify docstring.
(shell): Simplify.

doc/emacs/cmdargs.texi

@@ -654,7 +654,8 @@ Used by the Gnus package.
 @item SHELL
 @vindex SHELL@r{, environment variable}
 The name of an interpreter used to parse and execute programs run from
-inside Emacs.
+inside Emacs.  This is used to initialize the variable
+@code{shell-file-name} (@pxref{Single Shell}).
 @item SMTPSERVER
 @vindex SMTPSERVER@r{, environment variable}
 The name of the outgoing mail server.  This is used to initialize the

doc/emacs/misc.texi

@@ -810,6 +810,7 @@ to @command{gpg}.  This will output the list of keys to the
 buffer whose name is the value of @code{shell-command-buffer-name}.
 
 @vindex shell-file-name
+@cindex @env{SHELL} environment variable
   The above commands use the shell specified by the variable
 @code{shell-file-name}.  Its default value is determined by the
 @env{SHELL} environment variable when Emacs is started.  If the file
@@ -889,7 +890,6 @@ Subshells in different buffers run independently and in parallel.
 @vindex explicit-shell-file-name
 @cindex environment variables for subshells
 @cindex @env{ESHELL} environment variable
-@cindex @env{SHELL} environment variable
   To specify the shell file name used by @kbd{M-x shell}, customize
 the variable @code{explicit-shell-file-name}.  If this is @code{nil}
 (the default), Emacs uses the environment variable @env{ESHELL} if it

lisp/dired.el

@@ -1509,8 +1509,7 @@ see `dired-use-ls-dired' for more details.")
                     (script (format "ls %s %s" switches (cdr dir-wildcard)))
                     (remotep (file-remote-p dir))
                     (sh (or (and remotep "/bin/sh")
-                            (and (bound-and-true-p explicit-shell-file-name)
-                                 (executable-find explicit-shell-file-name))
+                            (executable-find shell-file-name)
                             (executable-find "sh")))
                     (switch (if remotep "-c" shell-command-switch)))
                ;; Enable globstar

lisp/obsolete/terminal.el

@@ -44,6 +44,7 @@
 ;;>>   more-processing enabled.
 
 (require 'ehelp)
+(require 'shell)
 
 (defgroup terminal nil
   "Terminal emulator for Emacs."
@@ -1056,12 +1057,6 @@ move to start of new line, clear to end of line."
 ;; This used to have `new' in it, but that loses outside BSD
 ;; and it's apparently not needed in BSD.
 
-(defcustom explicit-shell-file-name nil
-  "If non-nil, is file name to use for explicitly requested inferior shell."
-  :type '(choice (const :tag "None" nil)
-		 file)
-  :group 'terminal)
-
 ;;;###autoload
 (defun terminal-emulator (buffer program args &optional width height)
   "Under a display-terminal emulator in BUFFER, run PROGRAM on arguments ARGS.

lisp/shell.el

@@ -265,10 +265,11 @@ see the function `dirtrack-mode'."
   :group 'shell-directories)
 
 (defcustom explicit-shell-file-name nil
-  "If non-nil, is file name to use for explicitly requested inferior shell.
-When nil, such interactive shell sessions fallback to using either
-the shell specified in $ESHELL or in `shell-file-name'."
-  :type '(choice (const :tag "None" nil) file)
+  "If non-nil, the file name to use for explicitly requested inferior shells.
+When nil, such interactive shell sessions fall back to using the
+shell specified in either the environment variable \"ESHELL\" or
+`shell-file-name'."
+  :type '(choice (const :tag "Default" nil) file)
   :group 'shell)
 
 ;; Note: There are no explicit references to the variable `explicit-csh-args'.
@@ -748,16 +749,15 @@ Make the shell buffer the current buffer, and return it.
 
   (with-connection-local-variables
    ;; On remote hosts, the local `shell-file-name' might be useless.
-   (when (file-remote-p default-directory)
-     (if (and (called-interactively-p 'any)
+   (when (and (file-remote-p default-directory)
+              (called-interactively-p 'any)
               (null explicit-shell-file-name)
               (null (getenv "ESHELL")))
-         (set (make-local-variable 'explicit-shell-file-name)
-              (file-local-name
-	       (expand-file-name
-                (read-file-name
-                 "Remote shell path: " default-directory shell-file-name
-                 t shell-file-name))))))
+     (setq-local explicit-shell-file-name
+                 (file-local-name
+                  (expand-file-name
+                   (read-file-name "Remote shell path: " default-directory
+                                   shell-file-name t shell-file-name)))))
 
    ;; Rain or shine, BUFFER must be current by now.
    (unless (comint-check-proc buffer)

lisp/term.el

@@ -299,17 +299,13 @@
 ;; so it is important to increase it if there are protocol-relevant changes.
 (defconst term-protocol-version "0.96")
 
-(eval-when-compile (require 'ange-ftp))
-(eval-when-compile (require 'cl-lib))
-(require 'ring)
-(require 'ehelp)
+(eval-when-compile
+  (require 'ange-ftp)
+  (require 'cl-lib))
 (require 'comint) ; Password regexp.
-
-(declare-function ring-empty-p "ring" (ring))
-(declare-function ring-ref "ring" (ring index))
-(declare-function ring-insert-at-beginning "ring" (ring item))
-(declare-function ring-length "ring" (ring))
-(declare-function ring-insert "ring" (ring item))
+(require 'ehelp)
+(require 'ring)
+(require 'shell)
 
 (defgroup term nil
   "General command interpreter in a window."
@@ -393,11 +389,6 @@ by moving term-home-marker.  It is set to t if there is a
 (defvar-local term-line-mode-buffer-read-only nil
   "The `buffer-read-only' state to set in `term-line-mode'.")
 
-(defcustom explicit-shell-file-name nil
-  "If non-nil, is file name to use for explicitly requested inferior shell."
-  :type '(choice (const nil) file)
-  :group 'term)
-
 (defvar term-prompt-regexp "^"
   "Regexp to recognize prompts in the inferior process.
 Defaults to \"^\", the null string at BOL.