sched/headers: Move various ABI definitions to <uapi/linux/sched/types.h>

Move scheduler ABI types (struct sched_attr, struct sched_param, etc.) into
the new UAPI header.

This further reduces the size and complexity of <linux/sched.h>.

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>