openvpn-2fa: Import a prototype of an authenticator

openvpn-2fa: Import a prototype of an authenticator

This script runs aside of OpenVPN and connects to the management socket.
On the socket, OpenVPN will post any new clients trying to authenticate
which will be handled by the authenticator.

If a client has 2FA enabled, it will be challanged for the current token
which will then be checked in a second pass.

Clients which do not have 2FA enabled will just be authenticated no
matter what and tls-verify will have handled the rest.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>