Enable PreemptMode=suspend with PreemptType=qos

This only enables this configuration without providing the necessary
infrastructure to support it.
Add preempt_by_qos flag to select/cons_res
In select/cons_res add extra row to scheduling array if preempt/qos
  is used. This is used only when a job is started via preempting
  another job in the same partition, but a lower priority QOS.
Update preemption documentation