MAJOR: task: task scheduler rework.

MAJOR: task: task scheduler rework.

In order to authorize call of task_wakeup on running task:
- from within the task handler itself.
- in futur, from another thread.

The lookups on runqueue and waitqueue are re-worked
to prepare multithread stuff.

If task_wakeup is called on a running task, the woken
message flags are savec in the 'pending_state' attribute of
the state. The real wakeup is postponed at the end of the handler
process and the woken messages are copied from pending_state
to the state attribute of the task.

It's important to note that this change will cause a very minor
(though measurable) performance loss but it is necessary to make
forward progress on a multi-threaded scheduler. Most users won't
ever notice.