Add a trivial process.umask() implementation that returns 0.

I happened to need this method because I'm browserifying mkdirp, which happens to [use `process.umask()`](https://github.com/substack/node-mkdirp/blob/b98bedf92798ed73c87413eaf413d01dbe094a09/index.js#L19), along with [fake-fs](https://github.com/eldargab/node-fake-fs).