Clean up packet tracer, especially "clear trace"

There are multiple trace enablement schemes. It's easy to end up in
vlib_add_trace with tracing disabled insofar as the packet tracer is
concerned. When that happens, return the address of a per-system
dummy trace record.

Change-Id: I929391b8be4ed57e26e291afdc509a15f09a3160
Signed-off-by: Dave Barach <dave@barachs.net>