MINOR: mux-h2: implement MUX_CTL_GET_GLITCHES

This reports the number of glitches on a connection.

src/mux_h2.c

@@ -4623,6 +4623,9 @@ static int h2_ctl(struct connection *conn, enum mux_ctl_type mux_ctl, void *outp
 		tasklet_wakeup(h2c->wait_event.tasklet);
 		return 0;
 
+	case MUX_CTL_GET_GLITCHES:
+		return h2c->glitches;
+
 	default:
 		return -1;
 	}