Return name of function definition point is in, or nil.
Understands C, Lisp, LaTeX ("functions" are chapters, sections, ...),
Texinfo (@node titles) and Perl.
Other modes are handled by a heuristic that looks in the 10K before
point for uppercase headings starting in the first column or
identifiers followed by `:' or `='. See variables
`add-log-current-defun-header-regexp' and
`add-log-current-defun-function'.
Has a preference of looking backwards.
(fn)