dict_index_is_unique(): New function.

row_merge_rec_fits_to_block(), row_merge_store_rec_to_block():
Add const qualifiers to rec, offsets.

row_merge_select(): Rename to row_merge_cmp(), simplify the interface.

row_merge_sort_linked_list(): Change the return type to ibool.
Replace sec_offs1,sec_offs2 with offsets1,offsets2.  Replace offset_heap
with heap.  Define a func_exit label.

row_merge_block_merge(): Reduce the scope of rec_offsets.