Most users will connect to their server with `<username>` as their
username in order to configure their upstreams.
Multi-upstream can be unintuitive to them and should not be enabled on
that first connection that is usually used for upstream configuration.
Multi-upstream is instead a power-user feature that should be explicitly
enabled with a specific network suffix.
We reserve the network suffix `*` and use it a special case to mean that
it requests multi-upstream mode.
