(human_readable_base, output_units): Remove;

(human_readable_base, output_units): Remove;
replace with new variable output_block_size.  All uses changed.
(long_options, usage, main): Add --block-size.
(main, decode_switches): Use new human_block_size function to
initialize output block size consistently with other programs.

(print_header, show_dev): Shrink some columns and expand
others, to squeeze in support for today's larger filesystems.
(print_header): Print output block size using power-of-1024 SI format.
(df_readable): Coalesce last two args into one, for convenience.
All callers changed.
(main): Remove check for portable output format and larger
or human-readable block sizes.

From Paul Eggert.