Add find-file-asynchronously

Add find-file-asynchronously

* lisp/array.el (xor): Move function ...
* lisp/subr.el (xor): ... here.

* lisp/files.el (find-file-asynchronously): New defcustom.
(find-file-read-args): Use it.
(find-alternate-file-other-window, find-alternate-file): Use it.
Rename arg to ASYNC.
(find-file-with-threads, find-file, find-file-other-window)
(find-file-other-frame, find-file-existing, find-file--read-only)
(find-file-read-only, find-file-read-only-other-window)
(find-file-read-only-other-frame)
(find-alternate-file-other-window, find-file-noselect)
(find-file-literally): Rename arg to ASYNC.