By treating 'eshell-current-modifiers' as a hook, we can simplify much
of the code working with it and ensure that we call modifiers in a
more-correct order.
* lisp/eshell/em-dirs.el (eshell-expand-user-reference-1)
(eshell-expand-user-reference): Simplify. We now only get a single
argument.
(eshell-parse-user-reference):
* lisp/eshell/em-glob.el (eshell-add-glob-modifier):
* lisp/eshell/em-pred.el (eshell-parse-arg-modifier): Use 'add-hook'.
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