Only set Eshell execution result metavariables when non-nil

Only set Eshell execution result metavariables when non-nil

This simplifies usage of 'eshell-close-handles' in several places and
makes it work more like the docstring indicated it would.

* lisp/eshell/esh-io.el (eshell-close-handles): Only store EXIT-CODE
and RESULT if they're non-nil.  Also, use 'dotimes' and 'dolist' to
simplify the implementation.

* lisp/eshell/em-alias.el (eshell-write-aliases-list):
* lisp/eshell/esh-cmd.el (eshell-rewrite-for-command)
(eshell-structure-basic-command): Adapt calls to
'eshell-close-handles'.

* test/lisp/eshell/eshell-tests.el (eshell-test/simple-command-result)
(eshell-test/lisp-command, eshell-test/lisp-command-with-quote)
(eshell-test/for-loop, eshell-test/for-name-loop)
(eshell-test/for-name-shadow-loop, eshell-test/lisp-command-args)
(eshell-test/subcommand, eshell-test/subcommand-args)
(eshell-test/subcommand-lisp): Move from here...

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/simple-command-result, esh-cmd-test/lisp-command)
(esh-cmd-test/lisp-command-with-quote, esh-cmd-test/for-loop)
(esh-cmd-test/for-name-loop, esh-cmd-test/for-name-shadow-loop)
(esh-cmd-test/lisp-command-args, esh-cmd-test/subcommand)
(esh-cmd-test/subcommand-args, esh-cmd-test/subcommand-lisp): ... to
here.
(esh-cmd-test/and-operator, esh-cmd-test/or-operator)
(esh-cmd-test/for-loop-list, esh-cmd-test/for-loop-multiple-args)
(esh-cmd-test/while-loop, esh-cmd-test/until-loop)
(esh-cmd-test/if-statement, esh-cmd-test/if-else-statement)
(esh-cmd-test/unless-statement, esh-cmd-test/unless-else-statement):
New tests.

* doc/misc/eshell.texi (Invocation): Explain '&&' and '||'.
(for loop): Move from here...
(Control Flow): ... to here, and add documentation for other control
flow forms.