Ensure that Dhcp_client.input does not raise an exception
Ensure that Dhcp_client.input does not raise an exception

This breaks `mirage -t macosx` since the Vmnet interface sprays
all sorts of multicast packets which are not valid DHCP packets.

A future improvement to this layer is probably to listen on the
right DHCP ports, but this unbreaks OSX mirage for now.

haesbaert/charrua-core,haesbaert/hdhcp