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.

audio.c

@@ -985,7 +985,7 @@ int32_t search_for_suitable_configuration(unsigned int channels, unsigned int ra
         }
 #ifdef CONFIG_FFMPEG
         if (local_format == SPS_FORMAT_UNKNOWN) {
-          debug(3, "check for an next highest rate above %u with %u channels.", rate,
+          debug(3, "check for the next highest rate above %u with %u channels.", rate,
                 local_channels);
           unsigned int rate_pointer = 0;
           while ((rate_pointer < sizeof(rates) / sizeof(unsigned int)) &&
@@ -1003,7 +1003,7 @@ int32_t search_for_suitable_configuration(unsigned int channels, unsigned int ra
 
         if (local_format == SPS_FORMAT_UNKNOWN) {
           int rate_pointer = (int)(sizeof(rates) / sizeof(unsigned int) - 1);
-          debug(3, "check for an next lowest rate below %u with %u channels.", rate,
+          debug(3, "check for the next lowest rate below %u with %u channels.", rate,
                 local_channels);
           while ((rate_pointer >= 0) && (local_format == SPS_FORMAT_UNKNOWN)) {
             local_rate = rates[rate_pointer];