Consider actual support for file permissions

Saw a StackOverflow [question](http://stackoverflow.com/questions/24887693/testing-file-system-interaction-setting-file-permissions) about this.

It could be useful to have POSIX permissions for files in a file system actually affect behavior. For that to work, there'd also need to be some way to set a current user/group.

There are a variety of difficulties with this, though, such as:
- Do we try to make this work for Windows-like file systems as well?
- If there are other forms of authorization other file systems might use, is there some way to make this extensible so that a fake of one of those file systems can work properly?
- Management of context (e.g. user/group for current thread or other context) seems pretty unrelated to the file system and as such isn't something I really wanted to do.