Backport source links from 3.x.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.
diff --git a/Doc/library/asynchat.rst b/Doc/library/asynchat.rst
index 5096e4c..6f4af41 100644
--- a/Doc/library/asynchat.rst
+++ b/Doc/library/asynchat.rst
@@ -1,4 +1,3 @@
-
 :mod:`asynchat` --- Asynchronous socket command/response handler
 ================================================================
 
@@ -7,6 +6,9 @@
 .. moduleauthor:: Sam Rushing <rushing@nightmare.com>
 .. sectionauthor:: Steve Holden <sholden@holdenweb.com>
 
+**Source code:** :source:`Lib/asynchat.py`
+
+--------------
 
 This module builds on the :mod:`asyncore` infrastructure, simplifying
 asynchronous clients and servers and making it easier to handle protocols