interface: add minimal vpp pkt trace for error-drop

interface: add minimal vpp pkt trace for error-drop

Provide a minimal trace [ip4/ip6 src/dst address] for dropped pkts
when the user specifies "trace add error-drop XXXX", but does not
trace pkts from the original input node.

This is a wireshark dissector problem. Packets thrown at error-drop
may be well-formed, or not. VPP must not crash, no matter what.

The minimal trace capture and decode could be enhanced. Anyone
interested in doing that must consider all of the corner-cases
involved. This version should be at least somewhat useful.

Note that "pcap trace drop ..." - and the packet generator - seem like
the right tools to use when researching more complex issues.

Type: improvement

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I961ca133980ffa2a1e5707879a443b21442ed894