Deprecate sys.exitfunc in favor of the atexit module.

Per Guido, sys.exitfunc will be kept around for backwards compatability
but atexit will become the one preferred way to do it.

Doc/lib/libsys.tex

@@ -195,6 +195,7 @@ It is always available.
   module.  \note{The exit function is not called when the program is
   killed by a signal, when a Python fatal internal error is detected,
   or when \code{os._exit()} is called.}
+  \deprecated{2.4}{Use \refmodule{atexit} instead.}
 \end{datadesc}
 
 \begin{funcdesc}{getcheckinterval}{}