The Windows RNG turns out to only give about 16 bits at a time. This

The Windows RNG turns out to only give about 16 bits at a time. This
is (a) pretty feeble, and (b) means that although Net seeds transfer
between platforms and still generate the same game, there's a
suspicious discrepancy in the typical seed _generated_ by each
platform.
I have a better RNG kicking around in this code base already, so
I'll just use it. Each midend has its own random_state, which it
passes to new_game_seed() as required. A handy consequence of this
is that initial seed data is now passed to midend_new(), which means
that new platform implementors are unlikely to forget to seed the
RNG because failure to do so causes a compile error!

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@4187 cda61777-01e9-0310-a592-d414129be87e