sched: Introduce alternative task API.

sched: Introduce alternative task API.

In the current implementation struct task is public so users of this
structure can mess with internal scheduler details as they please. This
has led to many bugs and questionable code in the past. This commit
is the first step to overcome this design mistake. At the end of this
patch series struct task can be made private to sched.c.

This commit introduces the following new public functions:

	* task_register,
	* task_context,
	* sched_shutdown.

It also adds the new public task_info structure which carries the
information passed to the scheduler when a new task is registered. This
structure will stay public while struct task will become private once
all users have been converted.

task_register() is supposed to eventually replace register_task(). The
main difference of the two is that the new function returns a _pointer_
to a task structure which is allocated dynamically. Users are not
supposed to look at the fields of this pointer directly.

task_context() is a temporary helper which can be removed again at the
end of the series. Its sole purpose is to return the context pointer
which was passed at task register time as part of struct task_info.

The final new function, sched_shutdown(), deallocates the task
structures allocated during task_register() to cleanly shut down the
scheduler after all tasks have terminated.

All users need to be converted to the new API. This patch only converts
the stdout task though. The other tasks will be converted in subsequent
patches. The scheduler can tell if a task was registered using the
new API by means of the new ->owned_by_sched bit of struct task.
This boolean variable can also be removed after all tasks have been
converted.

Some users will need to query the exit status of a terminated
task. Hence we keep all tasks on the task list  after ->post_select()
returned negative but call neither ->pre_select() nor ->post_select()
any more for such tasks. This leads to the concept of zombie tasks.