Usability fix.  Makes the error message more helpful.

databases are associated with corruption problems, so I studied this code
carefully and ran some brutal stress tests.  I didn't find any bugs,
although it's unclear whether this code *intends* that __setitem__ can
leave the directory file out of synch with the data file (so
if a dumbdbm isn't properly closed, and the value of an existing key
was ever replaced, corruption is almost certain, where "corruption"
means the directory file is out of synch with the data file).

Added many comments and generally modernized the code.  Examples of the
latter:  we have better ways of reading a whole file line-by-line now;
eval() now tolerates a trailing newline; the %r format code can be used
to avoid explicit repr/backtick calls; and the code often broke tuples
into their components when it was clearer and faster to just leave them
as tuples.

Brett found that the tutorial didn't really explain what was happening
with exception targets.  Hopefully, this sheds some light on the subject.

Add API function simplefilter() that does not create or install
regular expressions to match message or module.  Extend the filters
data structure to store None as an alternative to re.compile("").

Move the _test() function to test_warnings and add some code to try
and avoid disturbing the global state of the warnings module.

- there's a weird variable name here (zimpimport), but I'll leave that
  for someone that's familiar with the ZIP import support

At this point, the problem appears particular to the OS/2 EMX port of
gdbm (which is at v1.7.3) - this combination produces a .pag file but
no .dir file.

A more sophisticated patch which checks magic numbers when dbm.library
indicates that dbm is linked to gdbm, and there is no .dir file, is
still attached to the above patch entry for reconsideration after 2.3
is released.

This checkin applies a workaround specific to the known failure case.

Expanded docs to have a quick start example showing how
to create and run tests.

supports pthreads

"Support IDLE Edit of .py/.pyw from idlelib"

until now: the inheritance of default values was the wrong way around.
This caused app bundles to get a type of "BNDL" instead of "APPL".
Apparently this is not a problem until you try to drag your app to
the dock.
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record tuple by name as well as index, to match the behaviour of
the pwd/grp extension modules for Unix.  These emulation modules
now pass test_pwd & test_grp.

installation location policies.

Use Py_AtExit instead of atexit so we are called during Py_Finalize()
rather than during DLL teardown.

 * Extended DB & DBEnv set_get_returns_none functionality to take a
   "level" instead of a boolean flag.  The boolean 0 and 1 values still
   have the same effect.  A value of 2 extends the "return None instead
   of raising an exception" behaviour to the DBCursor set methods.
   This will become the default behaviour in pybsddb 4.2.
 * Fixed a typo in DBCursor.join_item method that made it crash instead
   of returning a value.  Obviously nobody uses it.  Wrote a test case
   for join and join_item.