How should repl-options interact with multi-repl?

At the moment if you start a multi-repl and also pass `--repl-options` then the options you pass will be passed individually to each component.

An alternative would be to make `--repl-options` pass options at the top-level (along with the `-unit @` arguments).

In most cases both of these will do the same thing but some options like `-j` will only work if passed at the top-level.

Start by tracing how multi-repl assembles component arguments and how `--repl-options` is parsed and forwarded. Compare the current per-component behavior with top-level forwarding, especially for options such as `-j`; done requires a decided interaction model and corresponding implementation coverage.

Written by the indexing model from the issue text.