dd: summarize in --human-readable format too

dd: summarize in --human-readable format too

Problem reported by Linda Walsh in: http://bugs.gnu.org/17505
* NEWS: Document this.
* doc/coreutils.texi (dd invocation): Use a simpler script.
Adjust output example to match new behavior.
* src/dd.c (human_size): Remove.
All uses changed to use human_readable and ...
(human_opts): ... this new constant.
(abbreviation_lacks_prefix): New function.
(print_xfer_stats): Use it.  Output both --si and --human-readable
summaries, but only if they have prefixes.
* tests/dd/reblock.sh, tests/dd/stats.sh: Test new behavior.