MEDIUM: task: add a new flag TASK_RT to permit a task to skip the priority queue

MEDIUM: task: add a new flag TASK_RT to permit a task to skip the priority queue

For some very rare tasks that need to be woken up at an exact date (right
now the only known use case is haload's periodic stats collection), it's
currently difficult to guarantee the wake up date on a heavily loaded
run queue.

This patch introduces TASK_RT for real-time tasks. Right now, all it does
is modify __task_wakeup() to immediately switch to __tasklet_wakeup_*()
and effectively bypass the priority-based run queue. Doing it here has
the benefit of making sure that it automatically applies to tasks found
in the wait queue, and that it will also work for _task_drop_running().

For now nothing uses it. The doc was updated.