Prevent yet another staircase effect bug in the console device.

Prevent yet another staircase effect bug in the console device.

Even though I thought I fixed the staircase issue (and I was no longer
able to reproduce it), I got some reports of the issue still being
there. It turns out the staircase effect still occurred when
/dev/console was kept open while killing the getty on the same TTY
(ttyv0).

For some reason I can't figure out how the old TTY code dealt with that,
so I assume the issue has always been there. I only exposed it more by
merging consolectl with ttyv0, which means that the issue was present,
even on systems without a serial console.

I'm now marking the console device as being closed when closing the
regular TTY device node. This means that when the getty shuts down,
init(8) will open /dev/console, which means the termios attributes will
always be reset in this case.