Replace byte offsets of type dulint with block offsets of type ulint.
Avoid references to MERGE_BLOCK_SIZE.

Improve the language of some comments.

row_merge_insert_index_tuples(), row_merge_sort_linked_list_in_disk():
Make the offset a ulint.

row_merge_read(), row_merge_write(): Helper functions.  Return the
status of os_file_read() and os_file_write().  TO DO: check the status
in the callers.