openvpn: procd_set_param respawn

Makes sure that the openvpn instance gets restarted in case of a crash.

Intentional stops using /etc/init.d/openvpn stop will not result in
respawning. Anything else will, e.g. killall openvpn.

Signed-off-by: Lars Gierth <larsg@systemli.org>

SVN-Revision: 43886