Generalize universal-async-argument

Generalize universal-async-argument

* doc/lispref/files.texi (Visiting Functions):
Mention universal-async-argument.  Add @kindex and @findex.

* etc/NEWS: Mention 'universal-async-argument'.

* lisp/files.el (universal-async-argument): Move it to simple.el.
(find-file-read-args, find-alternate-file-other-window)
(find-alternate-file): Check for `universal-async-argument' variable.

* lisp/simple.el (universal-async-argument): New variable.
(universal-async-argument): Move function from files.el.  Don't
use an argument.  Toggle `universal-async-argument' variable.