sadf now displays file creation time

sadf -H, sadf -x and sadf -j now display the creation time of the
datafile. This time was available in the header of the datafile.
Time is displayed in UTC.

Signed-off-by: Sebastien GODARD <sysstat@orange.fr.fake>
Update DTD and XSD documents

Update DTD and XSD documents to take into account the creation time of
the datafile displayed by sadf -x.

Signed-off-by: Sebastien GODARD <sysstat@orange.fr.fake>