Fix logic bugs for max_rpc_cnt SchedulerParameters

Fix logic bugs for SchedulerParameters option of max_rpc_cnt.
Scheduling would be delayed for job arrays and
backfill scheduling would be disabled unless max_rpc_cnt > 0.
Update META for v2.6.9 tag
Start NEWS for v2.6.10
Merge branch 'slurm-2.6' into slurm-14.03

Conflicts:
	META
	NEWS