* lisp/eshell: Make backslash a no-op in front of normal chars

* lisp/eshell: Make backslash a no-op in front of normal chars

Fixes: debbugs:8531
* lisp/eshell/esh-arg.el (eshell-parse-argument-hook): Update comment.
(eshell-parse-backslash): Return escaped character after backslash
if it is special.  Otherwise, if the backslash is not in a quoted
string, ignore the backslash and return the character after; if
the backslash is in a quoted string, return the backslash and the
character after.

* test/automated/eshell.el (eshell-test/escape-nonspecial)
(eshell-test/escape-nonspecial-unicode)
(eshell-test/escape-nonspecial-quoted)
(eshell-test/escape-special-quoted): Add tests for new
`eshell-parse-backslash' behavior.