- Got rid of "struct idev".

- Use just the 32-bit numbers in the file_extras union.
- Re-ordered the items in the file_struct to avoid padding
  on systems with 64-bit pointers.
- Got rid of the "dir" union in the file_struct, making the
  two items into "extras": F_DEPTH() for the receiving side,
  and F_ROOTDIR() for the sending side.
- Added PTR_EXTRA_LEN define.
- Got rid of F_HL_IDEV() define.