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

(<inttypes.h>): Include if HAVE_INTTYPES_H.
("human.h"): Include.
(<limits.h>): Don't include; system.h does it now.
(INT_MAX): Remove.
(longdiff): Remove bogus definition that uses subtraction;
it gives the wrong answer when overflows occur.
(convert_blocks): Remove.
(output_units): New variable;
replaces booleans kilobyte_blocks and megabyte_blocks.
(human_readable_base): New variable.
(long_options, usage): Add -h or --human-readable and -H or --si.
(decode_switches): Adjust to renamed option variables.
Use -H if BLOCKSIZE is SI.  Add -h, -H.
(print_dir): Count blocks using uintmax_t, not int.
(gobble_file): Now returns uintmax_t, not int.
Don't convert blocks to 512 byte units, as this might overflow;
let caller handle the problem.  Deduce what caller will print
by invoking human_readable.
(print_long_format, print_file_name_and_frills): Don't assume
inode number, block count, file size fit in unsigned long.
Use human_readable to do block count conversion and to print
file sizes.
(prep_non_filename_text): Print decimal string for time if
localtime fails due to enormous time_t.