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.

lisp/eshell/em-alias.el

@@ -206,7 +206,7 @@ file named by `eshell-aliases-file'.")
       (let ((eshell-current-handles
 	     (eshell-create-handles eshell-aliases-file 'overwrite)))
 	(eshell/alias)
-	(eshell-close-handles 0))))
+	(eshell-close-handles 0 'nil))))
 
 (defsubst eshell-lookup-alias (name)
   "Check whether NAME is aliased.  Return the alias if there is one."

lisp/eshell/esh-cmd.el

@@ -541,9 +541,7 @@ implemented via rewriting, rather than as a function."
 	   	,(eshell-invokify-arg body t)))
 	     (setcar for-items (cadr for-items))
 	     (setcdr for-items (cddr for-items)))
-           (eshell-close-handles
-            eshell-last-command-status
-            (list 'quote eshell-last-command-result))))))
+           (eshell-close-handles)))))
 
 (defun eshell-structure-basic-command (func names keyword test body
 					    &optional else)
@@ -574,9 +572,7 @@ function."
   `(let ((eshell-command-body '(nil))
          (eshell-test-body '(nil)))
      (,func ,test ,body ,else)
-     (eshell-close-handles
-        eshell-last-command-status
-        (list 'quote eshell-last-command-result))))
+     (eshell-close-handles)))
 
 (defun eshell-rewrite-while-command (terms)
   "Rewrite a `while' command into its equivalent Eshell command form.

lisp/eshell/esh-io.el

@@ -254,6 +254,30 @@ a nil value of mode defaults to `insert'."
       (setq idx (1+ idx))))
   handles)
 
+(defun eshell-close-handles (&optional exit-code result handles)
+  "Close all of the current HANDLES, taking refcounts into account.
+If HANDLES is nil, use `eshell-current-handles'.
+
+EXIT-CODE is the process exit code (zero, if the command
+completed successfully).  If nil, then use the exit code already
+set in `eshell-last-command-status'.
+
+RESULT is the quoted value of the last command.  If nil, then use
+the value already set in `eshell-last-command-result'."
+  (when exit-code
+    (setq eshell-last-command-status exit-code))
+  (when result
+    (cl-assert (eq (car result) 'quote))
+    (setq eshell-last-command-result (cadr result)))
+  (let ((handles (or handles eshell-current-handles)))
+    (dotimes (idx eshell-number-of-handles)
+      (when-let ((handle (aref handles idx)))
+        (setcdr handle (1- (cdr handle)))
+	(when (= (cdr handle) 0)
+          (dolist (target (ensure-list (car (aref handles idx))))
+            (eshell-close-target target (= eshell-last-command-status 0)))
+          (setcar handle nil))))))
+
 (defun eshell-close-target (target status)
   "Close an output TARGET, passing STATUS as the result.
 STATUS should be non-nil on successful termination of the output."
@@ -305,32 +329,6 @@ STATUS should be non-nil on successful termination of the output."
    ((consp target)
     (apply (car target) status (cdr target)))))
 
-(defun eshell-close-handles (exit-code &optional result handles)
-  "Close all of the current handles, taking refcounts into account.
-EXIT-CODE is the process exit code; mainly, it is zero, if the command
-completed successfully.  RESULT is the quoted value of the last
-command.  If nil, then the meta variables for keeping track of the
-last execution result should not be changed."
-  (let ((idx 0))
-    (cl-assert (or (not result) (eq (car result) 'quote)))
-    (setq eshell-last-command-status exit-code
-	  eshell-last-command-result (cadr result))
-    (while (< idx eshell-number-of-handles)
-      (let ((handles (or handles eshell-current-handles)))
-	(when (aref handles idx)
-	  (setcdr (aref handles idx)
-		  (1- (cdr (aref handles idx))))
-	  (when (= (cdr (aref handles idx)) 0)
-	    (let ((target (car (aref handles idx))))
-	      (if (not (listp target))
-		  (eshell-close-target target (= exit-code 0))
-		(while target
-		  (eshell-close-target (car target) (= exit-code 0))
-		  (setq target (cdr target)))))
-	    (setcar (aref handles idx) nil))))
-      (setq idx (1+ idx)))
-    nil))
-
 (defun eshell-kill-append (string)
   "Call `kill-append' with STRING, if it is indeed a string."
   (if (stringp string)