workqueue: s/__create_workqueue()/alloc_workqueue()/, and add system workqueues

This patch makes changes to make new workqueue features available to
its users.

* Now that workqueue is more featureful, there should be a public
  workqueue creation function which takes paramters to control them.
  Rename __create_workqueue() to alloc_workqueue() and make 0
  max_active mean WQ_DFL_ACTIVE.  In the long run, all
  create_workqueue_*() will be converted over to alloc_workqueue().

* To further unify access interface, rename keventd_wq to system_wq
  and export it.

* Add system_long_wq and system_nrt_wq.  The former is to host long
  running works separately (so that flush_scheduled_work() dosen't
  take so long) and the latter guarantees any queued work item is
  never executed in parallel by multiple CPUs.  These will be used by
  future patches to update workqueue users.

Signed-off-by: Tejun Heo <tj@kernel.org>