Add fixme and default to none when srun and aprun are missing.

This can happen on an eslogin node.
Update chplenv launcher logic to print warning and always default to "none" instead of None.

Based on review comments from @Kyle-B
Merge branch 'master' of github.com:chapel-lang/chapel into launcher-default-fix
Merge pull request #138 from thomasvandoren/launcher-default-fix

Add fixme and default to none when srun and aprun are missing.