Relaxed min: option warning

When `min: 0` is given to `IO#getch` with `time:` option, it is
expected to return nil if timed out, and needed for source code
the compatibility with unixen platforms.

Author: nobu

ext/io/console/console.c

@@ -537,7 +537,13 @@ console_getch(int argc, VALUE *argv, VALUE io)
 	    timeout = rb_scheduler_timeout(&tv);
 # endif
 	}
-	if (optp->vmin != 1) {
+	switch (optp->vmin) {
+	  case 1: /* default */
+	    break;
+	  case 0: /* return nil when timed out */
+	    if (optp->vtime) break;
+	    /* fallthru */
+	  default:
 	    rb_warning("min option ignored");
 	}
 	if (optp->intr) {