Used by the routine at DESCRIBE_LOCATION.

Polls the whole keyboard through port $FE and returns once something is held, setting the border white on the way out. The game drops into it whenever a picture is finished -- the opening one, and each new location's after that -- before the next prompt, and a key pressed there is taken as "carry on" and not as a letter, which is why the first letter of a command typed while a picture was drawing went missing. (The title screen does not use this: it waits in its own loop at TITLE_WAIT.)