sched/headers: Move the JOBCTL_ defines and methods from <linux/sched.h> to <linux/sched/jobctl.h>

Only a small fraction of sched.h users actually utilizes these defines,
and they are not scheduler functionality in any case, so move them
into their separate header.

(Also make <linux/sched/jobctl.h> a self-contained header.)

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>