Implement fs.realpath()

[humphd]

See:

• http://nodejs.org/api/fs.html#fs_fs_realpath_path_cache_callback
• http://pubs.opengroup.org/onlinepubs/009695399/functions/realpath.html

Not sure how we want to deal with the fact that this requires the concept of a current working directory, in order to resolve things relative to . and ...

[modeswitch]

I think this kind of thing belongs in filer.io, which will handle VFS and shell stuff. Have a look at https://github.com/js-platform/filer.io/wiki/Design

[modeswitch]

Closing this as out-of-scope for filer. This, and other functions that are cwd-aware belong in filer.io.