It turns out diverting UDP sockets is pretty easy compared to TCP (which
makes it all the more embarrassing that they screwed up 'fwd' support for
UDP and not TCP, but oh well).  So let's use divert sockets instead of
transproxy for our DNS packets.

This is a little tricky because we have to do it all in firewall.py, since
divert sockets require root access, and only firewall.py has root access.