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.

pair_ap/pair.c

@@ -95,7 +95,7 @@ hash_init(enum hash_alg alg, HashCTX *c)
 }
 
 int
-hash_update(enum hash_alg alg, HashCTX *c, const void *data, size_t len)
+hash_update(__attribute__((unused)) enum hash_alg alg, HashCTX *c, const void *data, size_t len)
 {
 #if CONFIG_OPENSSL
   switch (alg)

pair_ap/pair_fruit.c

@@ -593,7 +593,7 @@ encrypt_ctr(unsigned char *ciphertext, int ciphertext_len,
 /* -------------------------- IMPLEMENTATION -------------------------------- */
 
 static int
-client_setup_new(struct pair_setup_context *handle, const char *pin, pair_cb add_cb, void *cb_arg, const char *device_id)
+client_setup_new(struct pair_setup_context *handle, const char *pin, __attribute__((unused)) pair_cb add_cb, __attribute__((unused)) void *cb_arg, __attribute__((unused)) const char *device_id)
 {
   struct pair_client_setup_context *sctx = &handle->sctx.client;
 
@@ -853,7 +853,7 @@ client_setup_result(struct pair_setup_context *handle)
 {
   struct pair_client_setup_context *sctx = &handle->sctx.client;
   char *ptr;
-  int i;
+  unsigned int i;
 
   // Last 32 bytes of the private key is the public key, so we don't need to
   // explicitly export that
@@ -867,13 +867,13 @@ client_setup_result(struct pair_setup_context *handle)
 
 
 static int
-client_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, pair_cb cb, void *cb_arg, const char *device_id)
+client_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, __attribute__((unused)) pair_cb cb, __attribute__((unused)) void *cb_arg, const char *device_id)
 {
   struct pair_client_verify_context *vctx = &handle->vctx.client;
   char hex[] = { 0, 0, 0 };
   size_t hexkey_len;
   const char *ptr;
-  int i;
+  unsigned int i;
 
   if (!is_initialized())
     return -1;
@@ -1024,7 +1024,7 @@ client_verify_response1(struct pair_verify_context *handle, const uint8_t *data,
 }
 
 static int
-client_verify_response2(struct pair_verify_context *handle, const uint8_t *data, size_t data_len)
+client_verify_response2(struct pair_verify_context *handle, __attribute__((unused)) const uint8_t *data, __attribute__((unused)) size_t data_len)
 {
   struct pair_client_verify_context *vctx = &handle->vctx.client;
   // TODO actually check response

pair_ap/pair_homekit.c

@@ -822,7 +822,7 @@ static void
 hexread(uint8_t *out, size_t out_len, const char *in)
 {
   char hex[] = { 0, 0, 0 };
-  int i;
+  unsigned int i;
 
   for (i = 0; i < out_len; i++, in+=2)
     {
@@ -1668,7 +1668,7 @@ static int
 client_setup_result(struct pair_setup_context *handle)
 {
   char *ptr;
-  int i;
+  unsigned int i;
 
   assert(sizeof(handle->result_str) >= 2 * sizeof(handle->result.client_private_key) + 2 * sizeof(handle->result.server_public_key) + 1);
 
@@ -1686,7 +1686,7 @@ client_setup_result(struct pair_setup_context *handle)
 }
 
 static int
-client_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, pair_cb cb, void *cb_arg, const char *device_id)
+client_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, __attribute__((unused)) pair_cb cb, __attribute__((unused)) void *cb_arg, __attribute__((unused)) const char *device_id)
 {
   struct pair_client_verify_context *vctx = &handle->vctx.client;
   size_t hexkey_len;
@@ -2888,7 +2888,7 @@ server_list_response(size_t *len, pair_list_cb cb, void *cb_arg)
 }
 
 static int
-server_list(uint8_t **out, size_t *out_len, pair_list_cb cb, void *cb_arg, const uint8_t *in, size_t in_len)
+server_list(uint8_t **out, size_t *out_len, pair_list_cb cb, void *cb_arg, __attribute__((unused)) const uint8_t *in, __attribute__((unused)) size_t in_len)
 {
   // Skip reading the request, it just has state = 1 and pair method =
   // PairingMethodListPairings