Move backward to the beginning of a defun.
With ARG, do it that many times. Negative ARG means move forward
to the ARGth following beginning of defun.
If search is successful, return t; point ends up at the beginning
of the line where the search succeeded. Otherwise, return nil.
When `open-paren-in-column-0-is-defun-start' is non-nil, a defun
is assumed to start where there is a char with open-parenthesis
syntax at the beginning of a line. If `defun-prompt-regexp' is
non-nil, then a string which matches that regexp may also precede
the open-parenthesis. If `defun-prompt-regexp' and
`open-paren-in-column-0-is-defun-start' are both nil, this
function instead finds an open-paren at the outermost level.
If the variable `beginning-of-defun-function' is non-nil, its
value is called as a function, with argument ARG, to find the
defun's beginning.
Regardless of the values of `defun-prompt-regexp' and
`beginning-of-defun-function', point always moves to the
beginning of the line whenever the search is successful.