sar: Stricter syntax check

"sar -1 -f" output was different than "sar -f -1" one.
So now enforce options check more aggressively: You cannot enter a day
offset (eg. -1) together with option -f (which tells here to use the
standard sa datafile).

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