numfmt: handle suffixes consistently with --{from,to}-unit

numfmt: handle suffixes consistently with --{from,to}-unit

* src/numfmt.c (unit_to_umax): Support SI (power of 10) suffixes
with the --from-unit and --to-unit options.  Treat suffixes like
is done with --from=auto, which for example will change the meaning
of --to-unit=G to that of --to-unit=Gi.  The suffix support was
previously undocumented and it's better to avoid the traditional
coreutils suffix handling in numfmt by default.
* doc/coreutils.texi: Document the new behavior.  Also fix a typo
mentioning {from,to}=units=.
* tests/misc/numfmt.pl: Adjust accordingly.
* NEWS: Mention the change in behavior.