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/xdrlib.rst b/Doc/library/xdrlib.rst
index 34d881d..e56650c 100644
--- a/Doc/library/xdrlib.rst
+++ b/Doc/library/xdrlib.rst
@@ -1,4 +1,3 @@
-
 :mod:`xdrlib` --- Encode and decode XDR data
 ============================================
 
@@ -10,6 +9,10 @@
    single: XDR
    single: External Data Representation
 
+**Source code:** :source:`Lib/xdrlib.py`
+
+--------------
+
 The :mod:`xdrlib` module supports the External Data Representation Standard as
 described in :rfc:`1014`, written by Sun Microsystems, Inc. June 1987.  It
 supports most of the data types described in the RFC.