When booted with RB_MULTIPLE, we had a notion of primary console where
all the rc messages went. However, this was to document what we did,
rather than being a highly desired feature.
While we retain that for input for now, for output put all messages to
the console to all the consoles.
Mirroring is controlled by the kern.console_mirror sysctl/tuneable,
it defaults to on, but can be turned off at any time.
Assisted-by: Claude Sonnet 5