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/tarfile.rst b/Doc/library/tarfile.rst
index 854ed74..5502adc 100644
--- a/Doc/library/tarfile.rst
+++ b/Doc/library/tarfile.rst
@@ -10,6 +10,9 @@
 .. moduleauthor:: Lars Gustäbel <lars@gustaebel.de>
 .. sectionauthor:: Lars Gustäbel <lars@gustaebel.de>
 
+**Source code:** :source:`Lib/tarfile.py`
+
+--------------
 
 The :mod:`tarfile` module makes it possible to read and write tar
 archives, including those using gzip or bz2 compression.