add option to not follow symlinks

[dbsr]

This adds a commandline option to not follow symlinks when resolving the cwd.

[b-ryan]

@dbsr this is a good idea, but I don't think your change is quite the right way to do it. It actually surprised me that the code is currently following symlinks. I think the default should be to not follow them. The get_valid_cwd also is doing more behavior than just following symlinks. I think the proper fix for this would be to return os.getenv['PDW'] in the get_valid_cwd function.