[ISSUE-95] Send replies from the right source IP address

Fixes #95

I didn't test this enough, but lets see what you have to say about it :)
I tried to keep it as simple as possible.

This has to be tested with IPv6. I guess that IPv6 is not supported yet (shouldn't be too hard to add IPv6 support though), but first lets make sure it doesn't break it.
I don't like the intrusiveness of this patch. I know you some how need to fit in the new stuff. If think it is better to add a completely new member this the server struct (or maybe hide in in udp so the srv.serve does not need to be changed. 

I would also like to contain this so it is only used when listening to 0.0.0.0 (so we can flag protect it).

And longer term: we should double check it v6 (this is v4 only I guess?), and (dare I say it), a test case would super super (but very hard to pull off)