Allow several mode: elements in the local variable section

* etc/NEWS (mode): Fall back on outline-mode in older Emacsen.
* lisp/files.el (hack-local-variables--find-variables): Use the
final mode: line (which is the same as having several mode: bits
in the header line.

etc/NEWS

@@ -2071,6 +2071,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
 
 Local variables:
 coding: utf-8
+mode: outline
 mode: emacs-news
 paragraph-separate: "[ 	]*$"
 end:

lisp/files.el

@@ -3978,8 +3978,7 @@ major-mode."
 	        (forward-line 1))
 	      (goto-char (point-min))
 
-	      (while (not (or (eobp)
-                              (and (eq handle-mode t) result)))
+	      (while (not (eobp))
 	        ;; Find the variable name;
 	        (unless (looking-at hack-local-variable-regexp)
                   (user-error "Malformed local variable line: %S"
@@ -4005,7 +4004,8 @@ major-mode."
 			   (not (string-match
 			         "-minor\\'"
 			         (setq val2 (downcase (symbol-name val)))))
-			   (setq result (intern (concat val2 "-mode"))))
+                           ;; Allow several mode: elements.
+                           (push (intern (concat val2 "-mode")) result))
 		    (cond ((eq var 'coding))
 			  ((eq var 'lexical-binding)
 			   (unless hack-local-variables--warned-lexical
@@ -4029,7 +4029,10 @@ major-mode."
 				         val)
                                    result))))))
 	        (forward-line 1)))))))
-    result))
+    (if (eq handle-mode t)
+        ;; Return the final mode: setting that's defined.
+        (car (seq-filter #'fboundp result))
+      result)))
 
 (defun hack-local-variables-apply ()
   "Apply the elements of `file-local-variables-alist'.

test/lisp/files-resources/file-mode-multiple

@@ -0,0 +1,5 @@
+Local variables:
+mode: text
+mode: test-mode-undef
+mode: outline
+end:

test/lisp/files-tests.el

@@ -1821,5 +1821,9 @@ Prompt users for any modified buffer with `buffer-offer-save' non-nil."
   ;; Check that the mode cookie doesn't override the explicit setting.
   (should (eq major-mode 'emacs-lisp-mode)))
 
+(ert-deftest files-test-set-mode-multiple ()
+  (find-file (ert-resource-file "file-mode-multiple"))
+  (should (eq major-mode 'outline-mode)))
+
 (provide 'files-tests)
 ;;; files-tests.el ends here