Give user some control over scheduling in tpool_submit()

[patrickmacarthur]

I envision the following three scenarios:

1. default case: This is the way that it is right now. A new thread would be spawned for the task if there are more threads available; otherwise, the task would be queued.
2. queued mode (e.g., task->flags & TPOOL_TASK_QUEUED): The task would be always queued unless there are no threads currently running in the pool, in which case a thread would be spawned for the task.
3. immediate exclusive mode (e.g., task->flags & TPOOL_TASK_IMMEDIATE): A new thread would be spawned for the task if there are more threads available; otherwise, tpool_submit() would return EBUSY.

Flag names aren't set in stone yet. If the two constants that I used above are used, then TPOOL_TASK_QUEUED and TPOOL_TASK_IMMEDIATE will be mutually exclusive (if used together, tpool_submit would return EINVAL).