Fix mdrun -nb auto -pme auto when GPUs are absent

Fix mdrun -nb auto -pme auto when GPUs are absent

The logic was flawed such that GPUs were "selected" for use even
though none had been detected. That led to the GPU behaviour of
avoiding using separate PME ranks.

Also made a minor fix to the logic for emulation. The new
interpretation of mdrun -gpu_id does not need to trigger an error when
GPU IDs have been supplied along with the emulation environmnet
variable.

Fixes #2315

Change-Id: I68da27c9bfef9f73b9dae4f04f196066d2efb1e2