Include human.h.

Include human.h.
(ROTATE_RIGHT): Remove; it was slow and the comment was
misleading anyway.
(bsd_sum_file): Avoid unsigned and long when they're not needed.
Compute total input bytes as uintmax_t, not long, so that the
code works even with files whose size does not fit in 'long'.
(sysv_sum_file): Likewise.
(sysv_sum_file): Do not reduce checksum until the end; this is
what System V does.