sched/headers: Move 'init_task' and 'init_thread_union' from <linux/sched.h>...

sched/headers: Move 'init_task' and 'init_thread_union' from <linux/sched.h> to <linux/sched/task.h>

'init_task' is really not part of core scheduler APIs but part of
the fork() interface between the scheduler and process management.

So move the declarations.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>