Just reuse the rng functionality. This improves the randomness.

Initialize seed with system time if no argument for RANDOMIZE is given. As the help text
states: "If Number is omitted, the generator uses the current value of the system timer".

Change-Id: I5fa46e8344b2402dff66a8db2449d43e2ca27d6d
Reviewed-on: https://gerrit.libreoffice.org/9349Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>