Win32 port: fix udp socket creation, merge args & simulate control pipe

Previous implementation will cause `EMSGSIZE` issue on Windows since it mistakenly sets the DF flag. Also it is not necessary and has the exact same function as `vpn_udp_alloc()` in `vpn.c` to bind the UDP port.

Issue reported by https://www.v2ex.com/t/138108#reply14.