(toplevel): Require button.

(toplevel): Require button.
(Man-header-file-path): New option.
(Man-synopsis-regexp, Man-files-regexp, Man-include-regexp)
(Man-file-name-regexp, Man-normal-file-prefix-regexp, Man-header-regexp)
(Man-normal-file-regexp): New variables.
(Man-mode-map): Inherited from `button-buffer-map'.  Don't define "\r" and
mouse-2 directly.  These key are defined in `button-buffer-map'.
(Man-xref-man-page, Man-xref-header-file, Man-xref-normal-file): New buttons.
`Man-xref-man-page' comes from woman.el.
(man-follow-mouse): Removed.
(Man-fontify-manpage): Use `Man-highlight-references' instead of calling
`add-text-properties' directly.
(Man-highlight-references, Man-highlight-references0): New functions.
(Man-view-header-file): New function.