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/Makefile.deps

@@ -228,6 +228,7 @@ LIBFILES= $(MANSTYLES) $(COMMONTEX) \
 	../lib/libstatvfs.tex \
 	../lib/libtty.tex \
 	../lib/libasyncore.tex \
+	../lib/libatexit.tex \
 	../lib/libcfgparser.tex
 
 # LaTeX source files for Macintosh Library Modules.

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}

Doc/lib/libatexit.tex

@@ -0,0 +1,59 @@
+\section{\module{atexit} ---
+         exit handlers}
+
+\declaremodule{standard}{atexit}
+\moduleauthor{Skip Montanaro}{skip@mojam.com}
+\sectionauthor{Skip Montanaro}{skip@mojam.com}
+\modulesynopsis{Register and execute cleanup functions.}
+
+The \module{atexit} module defines a single function to register
+cleanup functions.  Functions thus registered are automatically
+executed upon normal interpreter termination.
+
+Note: the functions registered via this module are 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.
+
+This is an alternate interface to the functionality provided by the
+\code{sys.exitfunc} variable.
+\withsubitem{(in sys)}{\ttindex{exitfunc}}
+
+\begin{funcdesc}{register}{func\optional{, *args\optional{, **kargs}}}
+Register \var{func} as a function to be executed at termination.  Any
+optional arguments that are to be passed to \var{func} must be passed
+as arguments to \function{register()}.
+
+At normal program termination (for instance, if
+\function{sys.exit()} is called or the main module's execution
+completes), all functions registered are called in last in, first out
+order.  The assumption is that lower level modules will normally be
+imported before higher level modules and thus must be cleaned up
+later.
+\end{funcdesc}
+
+
+\subsection{\module{atexit} Example \label{atexit-example}}
+
+The following simple example demonstrates how a module can initialize
+a counter from a file when it is imported and save the counter's
+updated value automatically when the program terminates without
+relying on the application making an explicit call into this module at
+termination.
+
+\begin{verbatim}
+try:
+    _count = int(open("/tmp/counter").read())
+except IOError:
+    _count = 0
+
+def incrcounter(n):
+    global _count
+    _count = _count + n
+
+def savecounter():
+    open("/tmp/counter", "w").write("%d" % _count)
+
+import atexit
+atexit.register(savecounter)
+\end{verbatim}
+

Doc/lib/libsys.tex

@@ -127,7 +127,9 @@ way to exit a program when an error occurs.
   This value is not actually defined by the module, but can be set by
   the user (or by a program) to specify a clean-up action at program
   exit.  When set, it should be a parameterless function.  This function
-  will be called when the interpreter exits.  Note: the exit function
+  will be called when the interpreter exits.  Only one function may be
+  installed in this way; to allow multiple functions which will be called
+  at termination, use the \refmodule{atexit} 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.
 \end{datadesc}