In checkdoc.el, get rid of the error-prone regex to find definition
forms, and use existing syntax-based navigation functions instead.
This fixes a corner case with one-argument `defvar' forms.
* lisp/emacs-lisp/checkdoc.el (checkdoc--next-docstring): New function.
(checkdoc-next-docstring, checkdoc-defun): Use it.
* test/lisp/emacs-lisp/checkdoc-tests.el (checkdoc-tests--next-docstring):
Add unit test.
See Bug#24998.
* lisp/emacs-lisp/checkdoc.el (checkdoc-defun-regexp): Don't require a
space before a argument list.
* test/lisp/emacs-lisp/checkdoc-tests.el (checkdoc-tests--bug-24998):
Add unit test.