Consolidate Eshell module loading/unloading code

Consolidate Eshell module loading/unloading code

This also adds the ability to suppress module loading/unloading
messages, which will be necessary to support running Eshell scripts as
batch scripts.

* lisp/eshell/esh-mode.el (eshell-mode): Move module
loading/initialization to...

* lisp/eshell/esh-module.el (eshell-load-modules)
(eshell-initialize-modules): ... here.
(eshell-module-loading-messages): New option.
(eshell-module--feature-name): Improve docstring.
(eshell-unload-modules): Display a real warning if unable to unload a
module.

* test/lisp/eshell/eshell-tests-helpers.el (with-temp-eshell)
(eshell-command-result-equal):
* test/lisp/eshell/eshell-tests-unload.el (load-eshell): Silence Eshell
loading messages.