Use os.tmpdir() for temp-dirs

As we do not want to support 0.6 as mentioned in https://github.com/isaacs/npm/pull/3498 I created another pull request which is reducing the code complexity. It is not node 0.6 compatible.

Feel free to pick the one you prefer for `osenv`

The former implementation preferred /home/{username}/tmp on unix
which caused errors for users that had different permissions on
their tmp-folder in their home, like root as owner from a previous
sudo-command.

The new implementation uses os.tmpdir()

This fixes: isaacs/npm#3470