Fix `gnus-union' so as to behave like `cl-union'

* lisp/gnus/gnus-group.el (gnus-group-prepare-flat):
Make gnus-union use `equal' to compare items in lists.

* lisp/gnus/gnus-util.el (gnus-union):
Make it behave like cl-union partially.

lisp/gnus/gnus-group.el

@@ -1396,7 +1396,8 @@ if it is a string, only list groups matching REGEXP."
       (gnus-group-prepare-flat-list-dead
        (gnus-union
 	not-in-list
-	(setq gnus-killed-list (sort gnus-killed-list 'string<)))
+	(setq gnus-killed-list (sort gnus-killed-list 'string<))
+	:test 'equal)
        gnus-level-killed ?K regexp))
 
     (gnus-group-set-mode-line)

lisp/gnus/gnus-util.el

@@ -1372,18 +1372,25 @@ Return the modified alist."
 
 (if (fboundp 'union)
     (defalias 'gnus-union 'union)
-  (defun gnus-union (l1 l2)
-    "Set union of lists L1 and L2."
+  (defun gnus-union (l1 l2 &rest keys)
+    "Set union of lists L1 and L2.
+If KEYS contains the `:test' and `equal' pair, use `equal' to compare
+items in lists, otherwise use `eq'."
     (cond ((null l1) l2)
 	  ((null l2) l1)
 	  ((equal l1 l2) l1)
 	  (t
 	   (or (>= (length l1) (length l2))
 	       (setq l1 (prog1 l2 (setq l2 l1))))
-	   (while l2
-	     (or (member (car l2) l1)
-		 (push (car l2) l1))
-	     (pop l2))
+	   (if (eq 'equal (plist-get keys :test))
+	       (while l2
+		 (or (member (car l2) l1)
+		     (push (car l2) l1))
+		 (pop l2))
+	     (while l2
+	       (or (memq (car l2) l1)
+		   (push (car l2) l1))
+	       (pop l2)))
 	   l1))))
 
 (declare-function gnus-add-text-properties "gnus"