Switch for Foreign UDP upstream

[riaqn]

Currently only TCP is used for foreign DNS. This is based on the observation that the most widely used proxies(shadowsocks) is STREAM-based(UDP is supported but is proxied via UDP, which is fucked by the QoS of many ISP, making it unusable).

However, some users may have fast and stable access to the UDP 53 port on the DNS server, which invalidates the above concern. Thus, we need to add a command-line switch to switch on foreign UDP upstream.