Use vpn_udp_alloc() to bind UDP port for win32

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.