bug#9279: df -h should use human_round_to_nearest in human_output_opts for more accurate results

[Paul Eggert]

tag 9279 notabug
thanks 

Thanks, but df always rounds up.  POSIX requires this for formats it specifies;
see <http://pubs.opengroup.org/onlinepubs/9699919799/utilities/df.html>
and look for "rounded".  for other formats, I thought it better to be
consistent.