Accept new naming convention for standard daily data files

sadc/sar/sadf: The standard daily data files may now be named saYYYYMMDD
instead of saDD, where YYYY stands for the current year, MM for the
current month, and DD for the current day.
Option -D has been added to sadc to tell it to use saYYYYMMDD instead of
saDD. This switch works only if no filename is explicitly given on the
command line with sadc.
The same option -D has been added to sar. It also tells it to use
saYYYYMMDD instead of saDD. This switch works only if no filename is
explicitly given on the command line *and* in conjunction with option
-o (ie. when sar is used to write data to files).
If you use sar to read data from a file without specifying this file's
name on the command line (eg. you enter just "sar") then sar will look
for the most recent of saDD and saYYYYMMDD and use it. sadf will do the
same.
If you use sar to write data to files (using its option -o) then it will
always use saDD data file unless option -D is used (in which case
saYYYYMMDD will be used). This makes it consistent with sadc behavior
(and I think it is safer not to let sar decide which file to write to).

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