Noticed recently that bitcount16() isn't 16-bit clean due to signed
shift right. It doesn't actually matter in the current code since
the input word only ever uses the bottom 9 bits, but if I ever
extended Mines to work in a triangular grid then all 16 bits might
be required. Fix this now, while I'm cleaning things up, so that it
won't bite me unexpectedly in future.

[originally from svn r6415]