audio/out: don't add special devices to --audio-device list

Since the list associated with --audio-device is supposed to enable
simple user-selection, it doesn't make much sense to include overly
special things like ao_pcm or ao_null in the list. Specifically,
ao_pcm is harmful, because it will just dump all audio to a file
named audiodump.wav in the current working directory. The user can't
choose the filename (it can be customized, but not through this
option), and the working directory might be essentially random,
especially if this is used from a GUI.

Exclude "strange" entries. We reuse the fact that there's already a
simple list ordered by auto-probe priority in order to avoid having to
add an additional flag. This is also why coreaudio_exclusive was moved
above ao_null: ao_null ends auto-probing and marks the start of
"special" outputs, which don't show up on the device, but we want
coreaudio_exclusive to be selectable (I think).

audio/out/ao.c

@@ -406,8 +406,8 @@ struct ao_device_list *ao_get_device_list(struct mpv_global *global)
         (struct ao_device_desc){"auto", "Autoselect device"});
     for (int n = 0; audio_out_drivers[n]; n++) {
         const struct ao_driver *d = audio_out_drivers[n];
-        if (d->encode)
-            continue;
+        if (d == &audio_out_null)
+            break; // don't add unsafe/special entries
         struct ao *ao = ao_alloc(true, global, NULL, (char *)d->name, NULL);
         if (!ao)
             continue;