Some DHCP messages are broadcast to 255.255.255.255, and binding to a
specific address filters those packets out. Instead, we have to translate
the "listen on ip:port" intent to "listen on interface:port", and filter
based on the interface that received the packet.
Fixes#27.
pixiecore: Plumb NewSnooperConn into Server as an advanced option.
pixiecore/cli: Plumb DHCPNoBind to the CLI.
Now, by default, Pixiecore will bind() to the DHCP server port in a way
that shows up in `netstat` by default. If you want to enable the "coexist
with my existing DHCP server" mode, you need to explicitly pass
--dhcp-no-bind.
