Fixes #4018: Move r30040 to v1.7 branch (resolve an unexpected behavior in hostfile allocations)
Fixes #4028: Move r30063 to v1.7 branch (Add missing var initialization)

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r30040 [[BR]]
Resolve an unexpected behavior in hostfile allocations. Now that we filter allocations to determine what will be used for mapping, let the initial global pool be the union of nodes from all sources (default hostfile, hostfiles, and dash-hosts). Each app will filter down to only those specified for it using its own hostfile and dash-host options.

cmr=v1.7.4:reviewer=jsquyres:subject=Resolve an unexpected behavior in hostfile allocations


r30063 [[BR]]
Add missing var initialization

cmr=v1.7.4:reviewer=ompi-gk1.7
Bring r30060 and r30062 over to the 1.7.4 branch, but keep the cmr open until we verify that this fixes the reported OOB problems

Refs #4026