Made site alias envvar file multi user aware.

The "drush use @site-alias2 feature does not work on a system with more than one user account using the feature.

From a real life experience today:

```bash
$ whoami
arne
$ ls -ld /tmp/drush-env
drwxr-xr-x 2 root root 4096 2013-09-04 18:25 /tmp/drush-env
```
Since another user has already created the `drush-env` folder I'm not able to set my environment.

This pull request incorporates the user ID in the folder name.