ovpnmain.cgi: Force NCP on clients

ovpnmain.cgi: Force NCP on clients

This change requires that all clients support NCP if they are set up
with a new connection. Existing clients remain supported using the
fallback cipher option.

This will result that connections with OpenVPN <= 2.3 cannot be set up
any more which is totally fine since that version is EOL.

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