Clean up file and hello messes.

adsbus/flow.c

@@ -0,0 +1,17 @@
+#include <unistd.h>
+
+#include "buf.h"
+
+#include "flow.h"
+
+bool flow_hello(int fd, struct flow *flow, void *passthrough) {
+	if (!flow->get_hello) {
+		return true;
+	}
+	struct buf buf = BUF_INIT, *buf_ptr = &buf;
+	flow->get_hello(&buf_ptr, passthrough);
+	if (!buf_ptr->length) {
+		return true;
+	}
+	return (write(fd, buf_at(buf_ptr, 0), buf_ptr->length) == (ssize_t) buf_ptr->length);
+}