(server) Ensure from address matches to address

[hildjj]

Since we're listening on "::", the kernel might choose a different from address for a reply than the original packet was addressed to. This will make 5-tuple binding fail.

http://stackoverflow.com/questions/3062205/setting-the-source-ip-for-a-udp-socket

Note: this might mean opening a separate v4 and v6 socket, which is annoying. Perhaps only worry about this for v6, where multiple addresses are likely more of an issue.