First cut on dentry support and permissions. First proof-of-concept...

First cut on dentry support and permissions. First proof-of-concept implementation for opendir, readdir, lookup, and releasedir. Implement next after paper is done.
dentry support is only partial and has to be restructured as the implementation of highlevel fuse is not transferable to lowlevel fuse. Readdir is significantly more complex but also much more lightweight which will help performance.