src: add ability to get/set effective uid/gid

Adds the following to process:

- `process.geteuid()`
- `process.seteuid(id)`
- `process.getegid()`
- `process.setegid(id)`

PR-URL: https://github.com/iojs/io.js/pull/1536


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