Added packet filtering support using $allow_address_ipv4/_ipv6 parameters

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Allowed access to ntpd from 127.0.0.1 and ::1 by default

This allows local ntp monitoring using tools such as ntpq.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>