[PATCH pkgs] Add button.el

Thanks!  A quick look seems all good to me.  Given our low level of
activity recently, any contribution is a good one.  I hate to say it,
but ship it, and we'll fix it later if needed.

Jerry James writes:
 > PATCH packages
 > 
 > There has been no response from Jeff Miller on the changes needed for
 > diary-lib.el in the calendar package, so I figured it out myself.  The
 > attached patch adds button.el from Emacs, which is already referred to
 > by a few of our packages.  The patch also removes existing workarounds
 > for the absence of button.el.