Use notation compatible with SI and with IEC 60027-2.

Use notation compatible with SI and with IEC 60027-2.
For example, --block-size=1MB now means --block-size=1000000,
whereas --block-size=1MiB now means --block-size=1048576.
A trailing `B' now means decimal, not binary; this is a silent change.
-H or --si now outputs the trailing 'B', for consistency with this.
Programs now output trailing 'K' (not 'k') to mean 1024.
New df, du short option -B is short for --block-size.
You can omit an integer `1' before a block size suffix,
e.g. `df -BG' is equivalent to `df -B 1G' and to `df --block-size=1G'.

Document the above.  Remove documentation
for obsolescent constructs MD, --kilobytes, -m or --megabytes.