rec_print_comp(): New function, sliced from rec_print_new().

rec_print_old(), rec_print_comp(): Print the untruncated length of the column.

row_merge_print_read, row_merge_print_write, row_merge_print_cmp:
New flags, to enable debug printout in UNIV_DEBUG builds.

row_merge_tuple_print(): New function for UNIV_DEBUG builds.

row_merge_read_rec(): Obey row_merge_print_read.

row_merge_buf_write(), row_merge_write_rec_low(),
row_merge_write_eof(): Obey row_merge_print_write.

row_merge_cmp(): Obey row_merge_print_cmp.