SP serial console doesn't detach stale faux-mgs clients on an idle console

[jgallagher]

faux-mgs allows us to attach to a gimlet's serial console via the SP. While faux-mgs is running, it ensures it's regularly sending serial console-related packets (either writes if there's data, or keeepalives if there isn't). On a clean exit, faux-mgs will detach itself from the console. But on a non-clean exit, the SP is responsible for eventually timing out the connection and allowing new attach requests. However, it looks like only check for staleness at

hubris/task/control-plane-agent/src/mgs_gimlet.rs

Lines 282 to 303 in 1d3f3d5

	// Do we have an attached MGS instance that hasn't gone stale?
	let (mgs_addr, sp_port) = match &self.attached_serial_console_mgs {
	Some(attached) => {
	// Check whether we think this client has disappeared
	let client_age_ms = sys_get_timer()
	.now
	.saturating_sub(attached.last_keepalive_received);
	if Duration::from_millis(client_age_ms)
	> SERIAL_CONSOLE_IDLE_TIMEOUT
	{
	self.usart.clear_rx_data();
	self.attached_serial_console_mgs = None;
	return None;
	}
	(attached.address, attached.port)
	}
	None => {
	// Discard any buffered data and reset any usart-related timers.
	self.usart.clear_rx_data();
	return None;
	}
	};

This is inside the code path for "the SP has serial console data to send to faux-mgs". If the console is idle (e.g., someone left it sitting at a shell prompt), the SP will never send data, so will never realize the faux-mgs client has gone away. This isn't a major problem, because faux-mgs can forcibly detach any previous session, but it is annoying because it makes it look like there might be someone else on the console when there isn't. A pretty easy fix would be to also check for staleness in serial_console_attach().