Fix Bug#21072 and rework `mark-defun'

Fix Bug#21072 and rework `mark-defun'

* test/lisp/progmodes/elisp-mode-tests.el (mark-defun-test-buffer):
  New variable
(mark-defun-no-arg-region-inactive)
(mark-defun-no-arg-region-active)
(mark-defun-arg-region-active)
(mark-defun-pos-arg-region-inactive)
(mark-defun-neg-arg-region-inactive, mark-defun-bob): Add tests for
the new `mark-defun'.

* lisp/emacs-lisp/lisp.el (beginning-of-defun--in-emptyish-line-p):
  New function.
(beginning-of-defun-comments): New function.
(mark-defun): Fix bug#21072, also rewrite large parts of `mark-defun'
to accept a numerical prefix argument.