VFS: _kern_open_dir(): Fix NULL path case

It would create an empty path buffer and pass that on to dir_open(),
which accepts NULL, but not empty paths.
packagefs: Fix handling of package domain paths

* The package directory path specified when mounting must be interpreted
  in the caller's I/O context. We did always interpret it in the
  kernel's I/O context, so that a relative path or a path in a chroot
  environment wouldn't work correctly. Now opening the directory is done
  in PackageDomain::Init() where we can at least guess the path's
  origin.
* We also normalize the path, though merely to get more conclusive debug
  output, since after opening the directory the path isn't used for
  anything else anymore.
* Make the "packages" mount parameter optional. If not specified, use
  the "packages" folder at our mount point.