Use short int consistently for unlocks. Check that the waits and unlocks don't overflow.

Two small changes:

• task->nr_unlock_tasks is a short int so we should be consistent with the types in scheduler_set_unlocks().
• When debugging, check that we don't overflow the number of wait or unlocks.

As I am about to introduce some gravity tasks that will have a lot of unlocks I would like to be safe first.