ttyplot.c: Get signal handling delay below 300ms for Ctrl+C

This is related to signal starvation with high pressure in stdin.
See the commit just before for more context.

Also not this part in `man 2 pselect`:
> Note that the timeout interval will be rounded up to the system
> clock granularity, and kernel scheduling delays mean that the
> blocking interval may overrun by a small amount.

So there is no guarantee for <=300ms.

ttyplot.c

@@ -499,7 +499,7 @@ int main(int argc, char *argv[]) {
         if (previous_parse_succeeded) {
             timeout.tv_nsec = 0;  // we may have more input pressing, let's not throttle it down
         } else {
-            timeout.tv_nsec = 500 * 1000 * 1000;  // <=500 milliseconds for a healthy clock display
+            timeout.tv_nsec = 200 * 1000 * 1000;  // <=500 milliseconds for a healthy clock display
         }
         const int select_ret = pselect(select_nfds, &read_fds, NULL, NULL, &timeout, &empty_sigset);