cmd/trace: add -d that prints parsed traces

cmd/trace: add -d that prints parsed traces

This is useful when debugging the tool.

Some tweaks on logging: log the webserver address, log.Print instead
of log.Printf when possible.

Change-Id: Iaf71b6523b40dc13795511784d48eacf0f5a396a
Reviewed-on: https://go-review.googlesource.com/59570
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>