Avoid shadowing variables in some Eshell command forms

Avoid shadowing variables in some Eshell command forms

* lisp/eshell/esh-cmd.el (eshell-rewrite-for-command): Make
'for-items' an uninterned symbol.
(eshell-as-subcommand): Correct docstring.
(eshell-do-command-to-value): Mark obsolete.
(eshell-command-to-value): Move binding of 'value' outside of the
macro's result, and remove call to 'eshell-do-command-to-value'.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/subcommand-shadow-value)
(esh-cmd-test/for-loop-for-items-shadow): New tests.
(esh-cmd-test/for-name-loop, esh-cmd-test/for-name-shadow-loop):
Rename to...
(esh-cmd-test/for-loop-name, esh-cmd-test/for-loop-name-shadow):
... these.