use /dev/urandom on unixes
and
rand_s() on windows

Change-Id: Iccd6e01706f16d82f23cda0f0a410953c32fbc02
Reviewed-on: https://gerrit.libreoffice.org/15473Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>