Allow unloading Eshell

Allow unloading Eshell

* lisp/eshell/em-extpipe.el (eshell-extpipe):
* lisp/eshell/esh-opt.el (eshell-opt): New groups.  Eshell uses these
to identify modules to unload.

* lisp/eshell/em-hist.el (eshell-hist-unload-hook):
* lisp/eshell/em-ls.el (eshell-ls-unload-hook):
* lisp/eshell/em-smart.el (eshell-smart-unload-hook):
* lisp/eshell/eshell.el (eshell-unload-hook): Make obsolete and move
to...

* lisp/eshell/em-smart.el (em-smart-unload-function):
* lisp/eshell/em-hist.el (em-hist-unload-function):
* lisp/eshell/em-ls.el (em-ls-unload-function):
* lisp/eshell/eshell.el (eshell-unload-function): ... these.

* lisp/eshell/esh-mode.el (eshell-mode-unload-hook):
* lisp/eshell/esh-module.el (eshell-module-unload-hook): Make
obsolete.

* lisp/eshell/em-ls (eshell-ls-enable-in-dired,
eshell-ls-disable-in-dired): New functions...
(eshell-ls-use-in-dired): ... use them.

* lisp/eshell/esh-module.el (eshell-module--feature-name,
eshell-unload-modules): New functions.
(eshell-unload-extension-modules): Use 'eshell-unload-modules'.

* lisp/eshell/eshell.el (eshell-unload-all-modules): Remove.

* test/lisp/eshell/eshell-tests-unload.el: New file.

* doc/misc/eshell.texi (Bugs and ideas): Remove item about unloading
Eshell not working.

* etc/NEWS: Announce this change (bug#61501).