client: --log-http shouldn't write to stderr

When using --log-http it would be nice if it didn’t write the log to stderr, but appended to a file (either well-defined name, or value specified by the user). Having that output appear mixed in with the normal output makes it harder to parse/extract. Often I might turn on this option when running a set of scripts so I can decide later if I want to debug it by looking at the http flows. Having all of this output mixed in with the normal output makes it much harder to process.