Collect filesystems stats only when sadc option "-S XDISK" is used

Make sadc collect filesystems statistics (those displayed by sar option
-F) only when option "-S XDISK" is used.
Filesystems are actually closer to partitions than to disks. So it makes
more sense for sadc to collect them when option "-S XDISK" is used
rather than option "-S DISK".
Update sadc and sar manual pages to reflect the change.