Fix Avahi mDNS backend

Fix Avahi polling:
 - use avahi_simple_poll_loop() in rend-avahi.c:rend_poll() instead
   of reinventing yet another type of wheel;
 - start RendezVous thread after daemonizing, so the thread actually
   runs after we've daemonized.

Also handle Avahi daemon restarts, as it's good practice and easy to do.

 ... And /that/, my friends, is how we do that ...

src/main.c

@@ -469,6 +469,12 @@ int main(int argc, char *argv[]) {
 
     runas = conf_alloc_string("general","runas","nobody");
 
+    if(!os_init(config.foreground,runas)) {
+      DPRINTF(E_LOG,L_MAIN,"Could not initialize server\n");
+      os_deinit();
+      exit(EXIT_FAILURE);
+    }
+
     if(config.use_mdns) {
         DPRINTF(E_LOG,L_MAIN,"Starting rendezvous daemon\n");
         if(rend_init(runas)) {
@@ -477,12 +483,6 @@ int main(int argc, char *argv[]) {
         }
     }
 
-    if(!os_init(config.foreground,runas)) {
-        DPRINTF(E_LOG,L_MAIN,"Could not initialize server\n");
-        os_deinit();
-        exit(EXIT_FAILURE);
-    }
-
     free(runas);
 
 #ifdef UPNP