call udp_tune() before bind()

This is because if you want SO_REUSEPORT you need the sockopt set first.
Without this patch a second copy of dnscrypt-wrapper will bail on trying to bind to an already open socket.
With this patch dnscrypt-wrapper at least has a chance to setsockopt() early enough to use the SO_REUSEPORT feature of kernel 3.9+

This is easy to fix for UDP, TCP is slightly less trivial because we're using libevent.
I'm going to propose TCP with a separate patch.

What do you think?
Maciej