Add conditional operator xor to subr.el

Suggested by Oleh Krehel and implemented by Basil Contovounesios in
the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00547.html

* lisp/array.el (xor): Move unused function from here...
* lisp/subr.el: ...to here, and improve.
* lisp/gnus/spam.el (spam-xor):
* lisp/play/5x5.el (5x5-xor):
* lisp/proced.el (proced-xor):
* lisp/progmodes/idlwave.el (idlwave-xor):
* lisp/vc/diff-mode.el (diff-xor): Define as obsolete aliases of,
and replace all uses with, xor.
* lisp/jsonrpc.el: Remove unused dependency on array.el.
* lisp/org/org.el (org-xor): Move from here...
* lisp/org/org-compat.el (org-xor): ...to here, as a compatibility
shim for xor.
* lisp/progmodes/idlw-shell.el (idlwave-shell-enable-all-bp):
* lisp/simple.el (exchange-point-and-mark):
* lisp/windmove.el (windmove-display-in-direction): Use xor.
* lisp/strokes.el (strokes-xor): Remove commented-out xor
implementation.

* doc/lispref/control.texi (Control Structures): Extend menu entry
for new combining condition.
(Combining Conditions):
* etc/NEWS (Lisp Changes): Document xor.

* test/lisp/subr-tests.el (subr-test-xor): New test.

lisp/subr.el

@@ -209,6 +209,14 @@ value of last one, or nil if there are none.
   (declare (indent 1) (debug t))
   (cons 'if (cons cond (cons nil body))))
 
+(defsubst xor (cond1 cond2)
+  "Return the boolean exclusive-or of COND1 and COND2.
+If only one of the arguments is non-nil, return it; otherwise
+return nil."
+  (declare (pure t) (side-effect-free error-free))
+  (cond ((not cond1) cond2)
+        ((not cond2) cond1)))
+
 (defmacro dolist (spec &rest body)
   "Loop over a list.
 Evaluate BODY with VAR bound to each car from LIST, in turn.