Changed setup.py's extend_user_env_posix to not rewrite every time.

I changed setup.py's extend_user_env_posix function.  This change means 2 things:

1.) extend_user_env_posix writes to ~/.bashrc, not ~/.profile.  It's more widly used and doesn't require a new login, just a new virtual terminal.
2.) extend_user_env_posix will check if it needs to write to ~/.bashrc, and only write if ~/.bashrc doesn't include an export statment for the current directory.