os: deprecate `tmpDir()` in favour of `tmpdir()`

`tmpdir()` was introduced as replacement 3 years ago in
3fe6aba5

PR-URL: https://github.com/nodejs/node/pull/6739


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>