--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.

Start at the CLI entry point for --log-http and trace how its HTTP logs are currently sent to stderr. Decide and document the output-file behavior, then verify that HTTP logs are separated from normal command output and remain available for later inspection.

Written by the indexing model from the issue text.