fix #K4097 - ^C during DECgraphics on tty

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.

win/tty/wintty.c

@@ -2225,6 +2225,16 @@ tty_putstr(winid window, int attr, const char *str)
         int suppress_history = (attr & ATR_NOHISTORY),
             urgent_message = (attr & ATR_URGENT);
 
+#if defined(ASCIIGRAPH) && !defined(NO_TERMS)
+        /* if ^C occurs, player is prompted with "Really quit?" and that
+           prompt is issued via tty_putstr(WIN_MESSAGE); if ^C happens
+           while writing DECgraphics chars, the prompt text would be
+           rendered as VT line-drawing characters unless we do this */
+        if (GFlag) {
+            graph_off();
+            GFlag = FALSE;
+        }
+#endif
         /* if message is designated 'urgent' don't suppress it if user has
            typed ESC at --More-- prompt when dismissing an earlier message;
            besides turning off WIN_STOP, we need to prevent current message