recent changes in 93c0742 reduced the msize returned by the server: this is still valid in 9p but not what the suite expected.

we can't call unlinkat with `.' as path, so the hackish solution (but also the only viable one I can see) is to unlinkat ../<dirname>. Furthermore, calling unlinkat(f->dir->fd, f->fpath) was incorrect because f->dir->fd already points to the directory.

I've been using the qid both as the 9p protocol element *and* as a wrapper with refcounting around a directory file descriptor. I can do better, let's split the qid struct into a "proper" qid and dir. Dir is now the wrapper with refcounting around the directory file descriptor and qid is just the 9p struct. fids are still associated with a dir, but now they store the directory name if they point to the directory. The previous pattern to detect if a fid is pointing to a dir thus changed: instead of assuming that fpath is "", look at the qid struct.