Improve correctness of Eshell sub-forms

This makes sure that we treat Eshell sub-forms (whether Lisp or command
forms) as values when appropriate, or as regular invocations.  This
requires a bit more explicit work, but helps to resolve some of the
surprising differences between Lisp and command forms in complex Eshell
statements.

* lisp/eshell/esh-cmd.el (eshell-subcommand-arg-values): Make obsolete.
(eshell-parse-lisp-argument): Don't add 'eshell-command-to-value' here.
(eshell-rewrite-sexp-command): Don't check for 'eshell-command-to-value
here'; instead check for 'eshell-lisp-command'.
(eshell-structure-basic-command): Check for 'eshell-lisp-command'.
(eshell-term-as-value): New function...
(eshell-rewrite-named-command, eshell-rewrite-for-command): ... call it.

* lisp/eshell/esh-arg.el (eshell-parse-special-reference):
* lisp/eshell/esh-io.el (eshell-strip-redirections):
* lisp/eshell/esh-var.el (eshell-prepare-indices): Call
'eshell-term-as-value'.

* test/lisp/eshell/esh-arg-tests.el
(esh-arg-test/special-reference/command-form):
* test/lisp/eshell/esh-cmd-tests.el (esh-cmd-test/for-loop-lisp-body)
(esh-cmd-test/while-loop-lisp-body)
(esh-cmd-test/if-else-statement-lisp-body): New tests.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/interp-var-indices-subcommand): Add another command to
test.

* doc/misc/eshell.texi (Control Flow): Update documentation.

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