audio: add option for falling back to ao_null

The manpage entry explains this.

(Maybe this option could be always enabled and removed. I don't quite
remember what valid use-cases there are for just disabling audio
entirely, other than that this is also needed for audio decoder init
failure.)

options/options.c

@@ -383,6 +383,7 @@ const m_option_t mp_opts[] = {
     OPT_SETTINGSLIST("ao-defaults", ao_defs, 0, &ao_obj_list),
     OPT_STRING("audio-device", audio_device, 0),
     OPT_STRING("audio-client-name", audio_client_name, 0),
+    OPT_FLAG("audio-fallback-to-null", ao_null_fallback, 0),
     OPT_CHOICE("force-window", force_vo, 0,
                ({"no", 0}, {"yes", 1}, {"immediate", 2})),
     OPT_FLAG("ontop", vo.ontop, M_OPT_FIXED),

options/options.h

@@ -81,6 +81,7 @@ typedef struct MPOpts {
     struct m_obj_settings *audio_driver_list, *ao_defs;
     char *audio_device;
     char *audio_client_name;
+    int ao_null_fallback;
     int force_vo;
     int softvol;
     float mixer_init_volume;