From SF patch #852334.

existed at the time atexit first got imported.  That's a bug, and this
fixes it.

Also reworked test_atexit.py to test for this too, and to stop using
an "expected output" file, and to test what actually happens at exit
instead of just simulating what it thinks atexit will do at exit.

Bugfix candidate, but it's messy so I'll backport to 2.2 myself.

pythonrun.c:  In Py_Finalize, don't reset the initialized flag until after
the exit funcs have run.
atexit.py:  in _run_exitfuncs, mutate the list of pending calls in a
threadsafe way.  This wasn't a contributor to bug 128475, it just burned
my eyeballs when looking at that bug.

added test script and expected output file as well
this closes patch 103297.
__all__ attributes will be added to other modules without first submitting
a patch, just adding the necessary line to the test script to verify
more-or-less correct implementation.

<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.