Make tasks gracefully exitable.

[dave]

Currently if a task falls out of its "main", it'll probably cause a system crash, since nothing decent is loaded in the link registers. The task should exit cleanly, unregistering itself from the scheduler, and should DTRT with regard to held semaphores (assert or unlock, not sure).

[dave]

Task exiting and wrapping up is implemented in 50e61bebba25. No semaphore checking is implemented yet.

[dave]

Semaphore checking pushed off to another ticket.