Although it looks like the sequencing of writing bs to the socket
and reusing the slice for the read test is guaranteed by there being
a receive from the socket in between, the race detector disagrees, and
found a race.
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.
