(<inttypes.h>): Include if HAVE_INTTYPES_H.

(<inttypes.h>): Include if HAVE_INTTYPES_H.
("human.h"): Include.
(LONGEST_HUMAN_READABLE_1K_BYTE_BLOCKS): Remove.
(human_readable_base): Renamed from human_blocks; value is now
zero or positive integer, not just zero or nonzero.
(output_units): New variable;
replaces booleans kilobyte_blocks and megabyte_blocks.
(long_options): Add --si or -H.
(print_header): Adjust to renamed option variables.
(human_readable_1k_blocks): Remove.
(show_dev): Count blocks using uintmax_t, not long.
Calculate percentages using double, not long; this still isn't
perfect as it suffers double rounding, but it's more likely to
round correctly in practice than using long did.
Adjust to renamed option variables.
Use new human_readable library function to format uintmax_t values.
(usage): Add -H, --si.
(main): Adjust to renamed option variables.
Use -H if BLOCKSIZE is SI.  Add -H.