Ensure function boundaries for threads state changes

Ensure function boundaries for threads state changes

Ensure threads are sleeping when leaving init_threads() and
the newly introduced put_threads_to_sleep().

Also ensure threads are not sleeping when leaving
wake_sleeping_threads().

As a side effect we now leave think() with all the threads
(but the main one) guaranteed to sleep. So when we enter
again in think(), after the opponent next move, we know
threads must be sleeping.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>