Add a sticky "PF_DEAD" task flag to keep track of dead processes.

Add a sticky "PF_DEAD" task flag to keep track of dead processes.

Use this to simplify 'finish_task_switch', but perhaps more
importantly we can use this to track down why some processes
seem to sometimes not die properly even after having been
marked as ZOMBIE. The "task->state" flags are too fluid to
allow that well.