(try_cursor_movement): Exclude header line from scroll

(try_cursor_movement): Exclude header line from scroll
margin at top of window.
(try_window_reusing_current_matrix): Calculate proper cursor position
after scrolling up with non-zero scroll margin, as the old cursor
position corresponds to value of PT before executing this command.
(try_window_id): Consider scroll margin at bottom of window too;
otherwise we fail to scroll when hl-line-mode is enabled.