* 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.