Clean up _bsddb.c: add a couple dozen missing Py_DECREF()'s, a handful of

missing PyObject_Del()'s, simplify some code by using Py_BuildValue()
instead of creating a tuple with items manually, stop clobbering builtin
exceptions in a few places, and guard against NULL-returning functions some
more.

This fixes 117 of the 780 (!?!#%@#$!!) reference leaks in test_bsddb3. I
ain't not done yet, although this review of 5kloc was just the easy part.