As it turns out, it's fairly difficult to do anything useful with DHCP
without this information, and the Conn implementations I can think of
all provide the information, so we might as well make it a requirement.
Conn is supposed to accept "" to mean "listen for DHCP on all interfaces",
but portableConn didn't correctly handle that.
And I got the comparison wrong on ifidx in the portable send function, so
packets got sent out entirely the wrong interface :(.
