package/openvpn: use start-stop-daemon to perform start/stop actions

Using 'start-stop-daemon' prevents multiple instances creation by
'/etc/init.d/S60openvpn start'.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>