Lot of changes here.

1. check_classic_mac_basic.yml — update to latest macOS-latest
2. docker-on-push-tag-or-pr — dependant updates
3. FFTConvolver — fix a compilation warning when the DEBUG flag is turned off
4. Reorganise some source files — e.g. move most of the metadata stuff into a subdirectory
5. Remove unused special-purpose AirPlay 2 RC event handler.
6. Reorganise source files and source code relating to the standard event handler.
7. Check configuration settings a little more carefully to offer more meaningful diagnostics.
8. Add password to AirPlay 2 and reorganise initialisation to set AirPlay 2 flags accordingly.  Also, remove redundant airplay_pin.
9. Remove redundant debug facility definitions from common.h
10. Make it possible to omit all the dacp stuff from a build.
11. Make it possible to omit metadata multicast from a build.
12. Add AP2-native metadata by setting bit 50 of features to true.
13. Improve stability when pre-empting an existing play session.
14. Fix a few compiler warnings in the pair_ap library.
15. Ensure that the audio format is known before starting synchronisation.
16. Improve handle_configure to set/clear a new config setting. Doesn’t seem to do anything…
17. Experimentally turn off bit 46 of AirPlay 2 features — seems to stop failing pair-verify attempts.

player.h

@@ -257,6 +257,7 @@ typedef struct {
 typedef struct {
   int connection_number;           // for debug ID purposes, nothing else...
   int is_playing;                  // set true by player_play, set false by player_stop
+  int input_format_is_valid;       // set when the input format is known and set in this structure
   unsigned int sync_samples_index; // for estimating the gap between the highest and lowest timing
                                    // error over the past n samples
   unsigned int sync_samples_count; // the array of samples is defined locally
@@ -272,7 +273,7 @@ typedef struct {
                              // otherwise
   int software_mute_enabled; // if we don't have a real mute that we can use
   int fd;
-  int authorized;   // set if a password is required and has been supplied
+  int authorized;   // set if a password is required and has been supplied or not required. Also always set in AirPlay 2 mode.
   char *auth_nonce; // the session nonce, if needed
   stream_cfg stream;
   SOCKADDR remote, local;
@@ -412,6 +413,7 @@ typedef struct {
                                // (will be unspecified if not build for AirPlay 2)
 
 #ifdef CONFIG_AIRPLAY_2
+  plist_t sessionPlist;
   char *airplay_gid; // UUID in the Bonjour advertisement -- if NULL, the group UUID is the same as
                      // the pi UUID
   airplay_t airplay_type; // are we using AirPlay 1 or AirPlay 2 protocol on this connection?
@@ -423,6 +425,8 @@ typedef struct {
   pthread_t rtp_ap2_control_thread;
   pthread_t rtp_realtime_audio_thread;
   pthread_t rtp_buffered_audio_thread;
+  pthread_mutex_t event_sender_mutex;
+  int event_channel_fd; // zero if closed, controlled by event_sender_mutex
 
   int ap2_event_receiver_exited;