interactive.c: disable remote access to the service pipe

Remote access to the service pipe is not needed and might be a potential attack vector.

For example, if an attacker manages to get credentials for a user which is the member of "OpenVPN Administrators" group on a victim machine, an attacker might be able to communicate with the privileged interactive service on a victim machine and start openvpn processes remotely.

* Reported by: Vladimir Tokarev <​vtokarev@microsoft.com>