Simplify the scheduling code.

Simplify the scheduling code.

	- Get rid of sched->select_ret. Tasks shouldn't care about the
	return value of the select call.

	- Kill task->private_data. Use container_of() instead.

	- Remove task->event_handler. It is never necessary and only
	makes the code more convoluted. The scheduler unregisters
	tasks as soon as either the pre_select or the post_select
	functions return an error.

	- Rename task->ret to task->error and get rid of a couple
	of error fields in other structs that usually only contained
	a copy of the task's error value.

This conversion likely introduces many bugs that have to be
shaken out in subsequent patches. Hopefully it will result in
less error-prone code in the long run.