branches/zip: Minor cleanup of merge_file_t and merge_block_header_t.

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.

include/row0merge.h

@@ -26,7 +26,7 @@ to this structure */
 
 struct merge_file_struct {
 	os_file_t	file;			/* File descriptor */
-	dulint		offset;			/* File offset */
+	ulint		offset;			/* File offset */
 	ulint		num_of_blocks;		/* Number of blocks */
 };
 
@@ -81,16 +81,16 @@ row_merge_insert_index_tuples(
 	dict_index_t*	index,		/* in: index */
 	dict_table_t*	table,		/* in: table */
 	os_file_t	file,		/* in: file handle */
-	dulint		offset);	/* in: offset where to start
+	ulint		offset);	/* in: offset where to start
 					reading */
 /*****************************************************************
 Merge sort for linked list in the disk. */
 
-dulint
+ulint
 row_merge_sort_linked_list_in_disk(
 /*===============================*/
 					/* out: offset to first block in
-					the list or ut_dulint_max in
+					the list or ULINT_UNDEFINED in
 					case of error */
 	dict_index_t*	index,		/* in: index to be created */
 	os_file_t	file,		/* in: File handle */

row/row0mysql.c

@@ -4522,12 +4522,10 @@ row_build_index_for_mysql(
 		which we have written at least one block */
 
 		if (merge_files[index_num].num_of_blocks > 0) {
-			dulint offset = ut_dulint_create(0, 0);
-
 			/* Merge sort file using linked list merge
 			sort for files. */
 
-			offset = row_merge_sort_linked_list_in_disk(
+			row_merge_sort_linked_list_in_disk(
 				index[index_num],
 				merge_files[index_num].file,
 				(int *)&error);
@@ -4535,8 +4533,7 @@ row_build_index_for_mysql(
 			if (error == DB_SUCCESS) {
 				error = row_merge_insert_index_tuples(
 					trx, index[index_num], new_table,
-					merge_files[index_num].file,
-					ut_dulint_zero);
+					merge_files[index_num].file, 0);
 			}
 
 			if (error != DB_SUCCESS) {