mympirun --dry-run should also show environment variables that are defined (or unset)

Especially with OpenMPI we set a couple of magic environment variables, we should expose those more clearly in the output of `--dry-run` (and `--debug`).

We should also make the unsetting of `$SLURM_EXPORT_ENV` a bit more verbose, since it makes debugging problems with commercial software that have an OpenMPI embedded easier.