Take into account alternate locations for sa data files

sadc, sar and sadf have been updated to take into account alternate
directory locations for sa daily data files (normally saved in
/var/log/sa directory).
When a directory (instead of a plain file) is used with sadc, sadf and
sar (with its options -f and -o) then it is now considered as the
location where the standard saDD data files are saved.

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