sched, cpuset: customize sched domains, docs

This patch introduces new feature of cpuset - sched domain customization.

This version provides a per-cpuset file 'sched_relax_domain_level' that
enable us to change the searching range of scheduler, which used to limit
how many cpus the scheduler searches at some schedule events, such as
wakening task and running out of runqueue.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>