(which-func-format): Make it risky-local-variable.

(which-func-format): Make it risky-local-variable.
(which-func-table): New var.
(which-func-current): Make it into a constant modeline spec.
(which-func-previous): Remove.
(which-func-update): Only update the selected window.
(which-func-update-1): Use the new var to allow the current
function to be different for a buffer shown in two windows.