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.
