Return EBUSY if we try to attach to a tty that is already being

snooped on.  This causes all kinds of Bad Things(tm) to happen since
closing one session will clobber state that's needed for the other
one.  This could theoretically be supported if the code was careful,
but until somebody implements that, preventing this will stop people
from unknowingly shooting themselves in the foot.

sys/dev/snp/snp.c

@@ -508,6 +508,8 @@ snpioctl(dev, cmd, data, flags, td)
 		tp = snpdevtotty(tdev);
 		if (!tp)
 			return (EINVAL);
+		if (tp->t_state & TS_SNOOP)
+			return (EBUSY);
 
 		s = spltty();