SF bug 434186: 0x80000000/2 != 0x80000000>>1

i_divmod:  New and simpler algorithm.  Old one returned gibberish on most
boxes when the numerator was -sys.maxint-1.  Oddly enough, it worked in the
release (not debug) build on Windows, because the compiler optimized away
some tricky sign manipulations that were incorrect in this case.
Makes you wonder <wink> ...
Bugfix candidate.