Added the atexit module and documentation from Skip Montanaro

<skip@mojam.com>.  Revisions to the markup to make it pass LaTeX, added
an index entry and a reference from the sys.exitfunc documentation.

This closes SourceForge patch #100620.

Doc/lib/lib.tex

@@ -73,6 +73,7 @@ and how to embed it in other applications.
 
 \input{libpython}		% Python Services
 \input{libsys}
+\input{libatexit}
 \input{libtypes}
 \input{libuserdict}
 \input{liboperator}