Disable setting implicit value of a job's cpus_per_task value

This removes logic added three years ago that would automatically set a job's cpus_per_task value in order to reset a job's mem_per_cpu value and scale the cpus_per_task by the same value. Equivalent logic did not exist in the step allocation logic. Just return an error instead. This change will be made in Slurm version 2.6, but this batch is made for version 2.5. The original patch introducing the problem is in commit: cc00cc70b9c90816afc511e0261e449857176332bug 352
Update switch documentation to clarify most calls are for a job step

Rather than a job
Srun to keep running after receiving unrecognized message

This can happen if something outside of Slurm opens the srun socket
and writes to it, since the data will not be of a form that Slurm
can decode.
Bug 354
Add job_submit plugin requiring a user specified time limit
Merge branch 'slurm-2.6'