For tty, if ^C interrupt occurred while the terminal was displaying
VT line drawing characters, it wouldn't finish updating the map and
switch back to regular characters, so the "Really quit?" prompt was
illegible.
Rather than muck about with the signal handler, just add a fixup to
tty_putstr() since prompting ultimately uses putstr(WIN_MESSAGE).
Reproducing the situation isn't straightforward; I didn't even try.
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