openvpn: T2381: bugfix rendering multiple openvpn-options from CLI

The CLI statement "set interfaces openvpn vtun10 openvpn-option '--tun-mtu 1500
--fragment 1300 --mssfix'" will render in vtun10.conf to:

--tun-mtu 1500 --fragment 1300 --mssfix

On startup OpenVPN complains about:

openvpn-vtun10: Options error: Unrecognized option or missing or extra

parameter(s) in vtun10.conf:76: tun-mtu (2.4.7)

The options must be split on -- to a new configuration line.