s390/3270: redraw screen on unsolicited device end

If a 3270 terminal is disconnected and later reconnected again,
it gets an unsolicited device end. This is currently ignored and
you have to hit the clear key to get the screen redrawn.
Add an automatic full redraw of the screen for this case.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>