Adds prototype NG driver apis

Includes an IPC system for out-of-process drivers so users won't have to recompile the registry to bring their own drivers. Also, drivers can potentially be written in any language with a functioning [libchan](https://github.com/docker/libchan) implementation.
This seems to add additional cruft, as it's just a one-liner function. There's no true difference between calling `d.subPath()` and `path.Join()`, IMO.