cb_param to I/O callbacks

[vsariola]

I'm trying to write LUA bindings of rocket and was not able to expose set_io_cb to the LUA-side for the following reasons:

1. Why does save_track not use the device io callbacks the same way as the read_track_data? This looks like an oversight and the sync_io_cb should have method write too:

rocket/lib/device.c

Line 374 in 147682c

static int save_track(const struct sync_track *t, const char *path)

2. I would need a way to pass userdata to the io callback (void *). I would need this for making a LUA-bindings of the rocket, as when I make the call back to LUA, I need a pointer to lua_State*, which gives access to the LUA virtual machine instance.

I think it could be even the same cb_param as is passed to the sync_update, but I'm not sure if it would be better to be a separate parameter:

rocket/lib/device.c

Line 544 in 147682c

sync_save_tracks(d);

The cb_param would have to be passed to any functions that do I/O.