path: refactor path.format() repeated code

PR-URL: https://github.com/nodejs/node/pull/5673


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

Conflicts:
	lib/path.js