Remove global variable opt_human_readable.

Remove global variable opt_human_readable.
(enum Output_units): Rename from output_size.
Rename size_* to Unit_*.
Add Unit_variable to correspond to --human-readable.
(convert_blocks): Remove now-unused definition.
(human_readable): Rename paramater n_bytes to n_blocks.
Adjust conversions to reflect fact that input is now number of
512-byte blocks, not bytes.
(print_size): New function.
(du_files): Use print_size instead of open coding it.
(count_entry): Always count in units of 512-byte blocks to delay.
This lets du accumulate totals corresponding to a terabyte before
overflowing 32-bit long int.
Use print_size instead of open coding it.