player: set thread name in opener thread

Somewhat helpful for debugging.

player/loadfile.c

@@ -28,6 +28,7 @@
 
 #include "osdep/io.h"
 #include "osdep/terminal.h"
+#include "osdep/threads.h"
 #include "osdep/timer.h"
 
 #include "common/msg.h"
@@ -829,6 +830,8 @@ static void *open_demux_thread(void *ctx)
 {
     struct MPContext *mpctx = ctx;
 
+    mpthread_set_name("opener");
+
     struct demuxer_params p = {
         .force_format = mpctx->open_format,
         .stream_flags = mpctx->open_url_flags,