* lisp/eshell/esh-cmd.el (eshell-rewrite-for-command): Don't modify the list

* lisp/eshell/esh-cmd.el (eshell-rewrite-for-command): Don't modify the list
in place.
(eshell-subcommand-arg-values, eshell-rewrite-named-command)
(eshell-rewrite-if-command, eshell-rewrite-for-command)
(eshell-structure-basic-command, eshell-rewrite-while-command)
(eshell-invokify-arg, eshell-parse-pipeline, eshell-parse-command)
(eshell-parse-subcommand-argument, eshell-parse-lisp-argument)
(eshell-trap-errors, eshell-do-pipelines, eshell-do-eval)
(eshell-do-pipelines-synchronously, eshell-eval-command):
Use backquotes and prefer setq to set.
(eshell-lookup-function, function-p-func, eshell-functionp): Remove.
(eshell-macrop): Use functionp.