devfs - implement mkdir/rmdir

devfs - implement mkdir/rmdir

* Implement mkdir and rmdir support to allow for custom user directories
  as used by LVM. The same policy applies as does to symlinks:
  user-created directories won't be garbage collected and are the only
  ones deletable by rmdir. One cannot delete automatically created
  directories and empty user-created directories won't be garbage
  collected automatically as happens with automatic dirs.