Make second arg to defvar-local optional

Make second arg to defvar-local optional

This change allows declaring a variable both special
and buffer-local like so:

    (defvar-local foo)

* lisp/subr.el (defvar-local): Make second argument optional.
* test/lisp/subr-tests.el (subr-test-defvar-local): New test.
* doc/lispref/variables.texi (Creating Buffer-Local): Document above change.
* etc/NEWS:

* lisp/mb-depth.el (minibuffer-depth-overlay):
* lisp/minibuf-eldef.el (minibuf-eldef-initial-input)
(minibuf-eldef-initial-buffer-length)
(minibuf-eldef-showing-default-in-prompt, minibuf-eldef-overlay):
* lisp/misc.el (list-dynamic-libraries--loaded-only-p):
* lisp/simple.el (minibuffer-history-isearch-message-overlay): Use
above new one-argument form of 'defvar-local'.

(cherry picked from commit ce03bf252ae9bff28a6def163cb3f4fa102e691a)