Added option -[0-9]+ to sadf to show data of that days ago

Make sadf consistent with sar. It's now possible to select the standard
system activity data file of, say 3 days ago, using a syntax like
"sadf -3".

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

Document sadf's new option -[0-9]+.

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