iso9660fs: add Rock Ridge extension

  - Port NetBSD code.
  - Add symbol link support for fs_lookup.
  - Add a fs call: fs_rdlink(30).
  - buf fix:
    . Can't read all the directory entries if spanning multi blocks.
      fixed function fs_getdents.
    . Can't lookup a inode if spanning multi blocks.
      fixed function search_dir
    . Can't do "cd .." correctly.
      fixed by Changing inode number of directory using the
      address of "." record whithin a directory as its inode number.