Option -c added to sadf: Upgrade an old sa datafile

This patch adds option -c to sadf. This option enables the user to
upgrade ("convert") an old system activity datafile (version 9.1.6 and
later) to the up-to-date format (11.1.1 as of today).
Enter "sadf -c old_datafile >new_datafile".

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