Squashed commit of the following:

Don't use strndup as old versions of OpenWrt don't seem to have it
    Update FREEBSD.md
    Fix comparison between old and new metadata strings by checking they have equal lengths as well.
    Remove a redundant definition to prevent use on an uninitialised copy, clean up some of the less-used backends
    Remove some unwanted and unused variable declarations from audio_dummy.c and audio_soundio.c
    Add the -fno-common flag to compilation.
    Treat the "mper" attribute as the 64-bit item that it is rather than a 32-bit item as hithereto. Output it as a hexadecimal number to correspond with the format of the track id obtained from AppleScript
    Make changes to that it compiles under gcc-10 with -fno-common
    Add a SIGCHLD handler to reap zombie processes generated after script invocations where wait_for_completion is set to "no".
    Update and rename CONTRIBUTING.md to REPORTING ISSUES.md
    Add a few more commands and clarify some text.
    Change from talking about a server to talking about a client. Technically, e.g. iTunes is a client of Shairport Sync.
    Store the UserAgent so as to recognise forked-daapd so as to always send a revision_number of 1 when asking for playerstatus of forked-daapd.
    Add or update some of the copyright notices
    Add the ability to set the volume directly to the D-Bus RemoteControl interface and to the MPRIS interface. Both use the recently-discovered ability to set the device_volume.
    Merge in Pieter De Gendt's work on resampling on the jack backend.
    Merge pull request #939 from pdgendt/feature/jack-soxr-resampling
    Add SetAirplayVolume to the D-Bus RemoteControl interface.
    Add SetVolume to the MPRIS interface.
    Hook up the Volume property in the MPRIS interface.
    Modify RemoteCommand in the D-Bus interface to return the HTTP status and response.
    Change the type of airplay_volume from int to double in the metadata hub.
    Add a few sample commands in the D-Bus document.

tinysvcmdns.c

@@ -114,8 +114,8 @@ uint8_t *join_nlabel(const uint8_t *n1, const uint8_t *n2) {
 
   s = malloc(len1 + len2 + 1);
   if (s) {
-    strncpy((char *)s, (char *)n1, len1);
-    strncpy((char *)s + len1, (char *)n2, len2);
+    memcpy((char *)s, (char *)n1, len1);
+    memcpy((char *)s + len1, (char *)n2, len2);
     s[len1 + len2] = '\0';
   } else {
     die("can not allocate memory for \"s\" in tinysvcmdns");
@@ -200,7 +200,7 @@ uint8_t *create_label(const char *txt) {
   s = malloc(len + 2);
   if (s) {
     s[0] = len;
-    strncpy((char *)s + 1, txt, len);
+    memcpy((char *)s + 1, txt, len);
     s[len + 1] = '\0';
   } else {
     die("can not allocate memory for \"s\" 2 in tinysvcmdns.");
@@ -222,7 +222,7 @@ uint8_t *create_nlabel(const char *name) {
   if (label == NULL)
     return NULL;
 
-  strncpy((char *)label + 1, name, len);
+  memcpy((char *)label + 1, name, len);
   label[len + 1] = '\0';
 
   p = label;
@@ -1466,7 +1466,7 @@ int close_pipe(int s) {
 
 // main loop to receive, process and send out MDNS replies
 // also handles MDNS service announces
-static void main_loop(struct mdnsd *svr) {
+void* main_loop(struct mdnsd *svr) {
   fd_set sockfd_set;
   int max_fd = svr->sockfd;
   char notify_buf[2]; // buffer for reading of notify_pipe
@@ -1568,6 +1568,7 @@ static void main_loop(struct mdnsd *svr) {
   close_pipe(svr->sockfd);
 
   svr->stop_flag = 2;
+  return NULL;
 }
 
 /////////////////////////////////////////////////////
@@ -1723,7 +1724,7 @@ struct mdnsd *mdnsd_start() {
   pthread_attr_init(&attr);
   pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
 
-  if (pthread_create(&tid, &attr, (void *(*)(void *))main_loop, (void *)server) != 0) {
+  if (pthread_create(&tid, &attr, (void * (*)(void *))&main_loop, (void *)server) != 0) {
     pthread_mutex_destroy(&server->data_lock);
     free(server);
     return NULL;