time(NULL) is a poor seed. It is quite predictable and
multiple instance starting in the same second will get the same seed
and therefore the same pseudo random number sequence

Use std::random_device, witch is meant to provide 'true' random
data.. mix time(NULL) just in case the std implementation is crappy.

PS: sadly std::random_device.entropy() cannot be relied on
as clang and gcc are known to return 0 despite their random_device
being non-deterministic, hence the prophylactic systematic
mixing with time(null)

Change-Id: I44dab9970f8f0388dc1c99e9816d49d1afbecf18
Reviewed-on: https://gerrit.libreoffice.org/15591Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>