- The function mount_entry_create_aufs_dirs() moves from conf.c to
  lxcaufs.{c,h} where it belongs.
- In accordance with the "aufs_" prefix naming scheme for functions associated
  with lxcaufs.{c,h} mount_entry_create_aufs_dirs() becomes aufs_mkdir().
- Add aufs_get_rootfs() which returns the rootfs for an aufs lxc.rootfs.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>