Improve correctness of 'eshell-quote-argument'

Improve correctness of 'eshell-quote-argument'

* lisp/eshell/esh-arg.el (eshell-quote-argument): Mention that this
function is for use within Eshell buffers.
(eshell-quote-backslash): Properly quote newlines.

* lisp/eshell/em-unix.el (eshell/cat, eshell/du): Throw
'eshell-external' instead; that's what it's here for.

* test/lisp/eshell/esh-proc-tests.el (esh-proc-test-quote-argument):
Remove.
(esh-proc-test/emacs-command):
* test/lisp/eshell/esh-var-tests.el (esh-var-test/path-var/set)
(esh-var-test/path-var/set-locally): Use 'eshell-quote-argument'.

* test/lisp/eshell/em-unix-tests.el (em-unix-test/compile/interactive):
Use 'shell-quote-argument' (Note: *not* 'eshell-...').

(cherry picked from commit 32a75ecc73b78ad922b2ae66d30b907b13e19cb8)