- Made the sock_{in,out} variables non-static.

- Added hlink_list, a FIFO list of finished hard-link items.
- Made get_redo_num() check for finished hard-link items and
  call the generator when they are found.  This ensures that
  we finish all the hard-link items by the time the MSG_DONE
  is read and returned to the generator.
- Added get_hlink_num() to read the new hlink_list.