Say "printing not supported", not just "not supported", for DLT_ values

Say "printing not supported", not just "not supported", for DLT_ values
for which we don't have a print routine; you *can* use tcpdump to
capture traffic with those DLT_ values, as long as you use "-w" so that
we don't interpret the contents of the packet, we just blindly write the
raw packet data to the capture file.