ox: Fix 88457acff411eca9b32c04b4bc100ef094a23c5c

* lisp/ox.el (org-export-async-start): Limit first argument to lambda
  expressions.
* lisp/ox-publish.el (org-publish, org-publish-all,
  org-publish-current-file): Replace `ignore', per limit stated above.

Due to a hack allowing to provide quasi-quoted lambda expressions,
symbols are not allowed as result handler.  This limitation is not
much of a problem as `org-export-async-start' is only meant to be used
internally.

lisp/ox-publish.el

@@ -881,7 +881,7 @@ publishing will be done asynchronously, in another process."
 			  ;; project is still a string here.
 			  (list (assoc project org-publish-project-alist)))))
     (if async
-	(org-export-async-start 'ignore
+	(org-export-async-start (lambda (results) nil)
 	  `(let ((org-publish-use-timestamps-flag
 		  (if ',force nil ,org-publish-use-timestamps-flag)))
 	     (org-publish-projects ',project-alist)))
@@ -899,7 +899,7 @@ optional argument ASYNC, publishing will be done asynchronously,
 in another process."
   (interactive "P")
   (if async
-      (org-export-async-start 'ignore
+      (org-export-async-start (lambda (results) nil)
 	`(progn
 	   (when ',force (org-publish-remove-all-timestamps))
 	   (let ((org-publish-use-timestamps-flag
@@ -921,7 +921,7 @@ asynchronously, in another process."
   (interactive "P")
   (let ((file (buffer-file-name (buffer-base-buffer))))
     (if async
-	(org-export-async-start 'ignore
+	(org-export-async-start (lambda (results) nil)
 	  `(let ((org-publish-use-timestamps-flag
 		  (if ',force nil ,org-publish-use-timestamps-flag)))
 	     (org-publish-file ,file)))

lisp/ox.el

@@ -5479,8 +5479,9 @@ to `:default' encoding. If it fails, return S."
 (defmacro org-export-async-start  (fun &rest body)
   "Call function FUN on the results returned by BODY evaluation.
 
-BODY evaluation happens in an asynchronous process, from a buffer
-which is an exact copy of the current one.
+FUN is an anonymous function of one argument.  BODY evaluation
+happens in an asynchronous process, from a buffer which is an
+exact copy of the current one.
 
 Use `org-export-add-to-stack' in FUN in order to register results
 in the stack.
@@ -5532,7 +5533,7 @@ and `org-export-to-file' for more specialized functions."
            ;; Register running process in stack.
            (org-export-add-to-stack (get-buffer ,proc-buffer) nil ,process)
            ;; Set-up sentinel in order to catch results.
-           (let ((handler ',fun))
+           (let ((handler ,fun))
              (set-process-sentinel
               ,process
               `(lambda (p status)