Send and recv all data when necessary

Expose net_recv_all() and net_send_all(), equivalent of net_recv() and
net_send(), but that waits/retries until the requested length has been
transferred.

Use these new functions where it was (wrongly) assumed that the
requested length had been transferred.

app/src/controller.c

@@ -47,7 +47,7 @@ static SDL_bool process_event(struct controller *controller, const struct contro
     if (!length) {
         return SDL_FALSE;
     }
-    int w = net_send(controller->video_socket, serialized_event, length);
+    int w = net_send_all(controller->video_socket, serialized_event, length);
     return w == length;
 }