iostat: JSON output now depends on options used

iostat: JSON output now depends on options used

Make JSON output take into account the options used with iostat.

This fixes a problem where some fields had a name like rkB/s or wkB/s
even when data were expressed in MB. In this example, the name will now
be rMB/s or wMB/s when data are expressed in MB.

This patch renames several fields, breaking backward compatibility that
I first hoped to keep. However all fields are now consistent with
iostat's standard report.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>