(print_header): Use primitives from inttostr.h, not

(print_header): Use primitives from inttostr.h, not
human.h, to print large numbers simply.

(human_output_opts): New var.
(output_block_size): Now uintmax_t, not int, to handle larger
block sizes.  All uses changed.

(print_header): In the header line, prefer SI to human
representation if it's shorter; if neither is shorter, try to
intuit what the user would prefer.