os: remove trailing slash from os.tmpdir()

This commit makes `os.tmpdir()` behave consistently on all platforms. It
changes `os.tmpdir()` to always return a path without trailing slash.

Semver: major
Fixes: https://github.com/iojs/io.js/issues/715
PR-URL: https://github.com/iojs/io.js/pull/747


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>