Support displaying function name in the header line

Support displaying function name in the header line

In some languages, the function name as displayed in the mode-line by
which-func-mode can be quite long.  It's useful to be able to display
it in the header-line instead.  Let's support that.

* lisp/progmodes/which-func.el (which-func-display)
(which-func--use-header-line, which-func--use-mode-line):
Add. (Bug#64533)
(which-func-try-to-enable): Support 'which-func--use-header-line'.
(which-func--disable): Add, to support 'which-func--use-header-line'.
(which-func-ff-hook, which-func-update-1): Use 'which-func--disable'.