Fix for node reboot/down state

In your node_mgr fix to keep rebooted nodes down (commit 9cd15dfe), you
forgot to consider the case of nodes that are powered up but are responding after
ResumeTimeout seconds (the maximum time permitted). Such nodes are
marked DOWN (because they didn't respond within ResumeTimeout seconds) than
should become silently available when ReturnToService=1 (as stated in the slurm.conf manual)

With your modification when such nodes are finally responding, they are seen as
rebooted nodes and remain in the DOWN state (with the new reason: Node
unexpectedly rebooted) even when ReturnToService=1 !

My patch to obtain the correct behaviour: