Fixes #4787: Move r32229 to v1.8 branch (Get the blasted singleton comm_spawn)

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r32229 [[BR]]
Get the blasted singleton comm_spawn working again. There remain problems with the Slurm interaction in this use-case as the PMI components (if configured to build) try to run even when a Slurm allocation hasn't been made, but I leave that to someone else to resolve. I did, however, tell the Slurm ess to quit interfering with applications launched in this use-case by ORTE daemons, so things do work when inside a Slurm allocation.

Also discovered that the rsh launcher is not picking up --enable-orterun-prefix-by-default when invoked during singleton comm_spawn, but I was unable to see why that was happening and ran out of time.

cmr=v1.8.2:reviewer=rhc