Fix so we do not get warnings when running on system without CUDA software installed and CUDA-aware compiled in.
Correctly add support for the "env" MPI_Info key during comm_spawn, update the "map-by", "rank-by", and "bind-to" Info key behaviors to match the new mapping/ranking/binding system, and update all docs and comments to match.

Fix comm_spawn on a single host - with the new default mapping scheme, we were incorrectly computing the number of procs to put on the node.

Refs #4003
Fix return value.