Rework "async" toggling of file visiting commands

Rework "async" toggling of file visiting commands

* doc/emacs/files.texi (Visiting):
* doc/lispref/files.texi (Visiting Functions):
* etc/NEWS: Use 'execute-file-commands-asynchronously' rather than
'find-file-asynchronously'.

* lisp/files.el (universal-async-argument): New defun.  Bind it
to "C-x &".
(find-file, find-file-other-window, find-file-other-frame)
(find-file-existing, find-alternate-file-other-window)
(find-alternate-file, find-file-literally): Adapt docstring.
(execute-file-commands-asynchronously): Rename from
`find-file-asynchronously'.
(find-file-read-args, find-alternate-file-other-window)
(find-alternate-file): Use it.