env clearing - clear better and in tmws as well.

- Move environment clearing from startws into a function in gtws.sh
- Add environment clearing to tmws, because the tmux server inherits the
  environment of the shell starting the first tmux session, so it will
  have a potentially full workspace environment in it.
- Restore PATH to it's original status
- Clear the prompt stuff
- Clear CDPATH
- Clear PYTHONPATH and LD_LIBRARY_PATH
- Clear SDIRS