set default listen() backlog to 511 on all platforms except FreeBSD
diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 0e732cf..f8bd6b9 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -2219,7 +2219,7 @@
 #endif
         ls->family = AF_INET;
 
-        ls->conf.backlog = -1;
+        ls->conf.backlog = NGX_LISTEN_BACKLOG;
         ls->conf.rcvbuf = -1;
         ls->conf.sndbuf = -1;
     }
@@ -2570,7 +2570,7 @@
     ls->port = u.port;
     ls->file_name = cf->conf_file->file.name;
     ls->line = cf->conf_file->line;
-    ls->conf.backlog = -1;
+    ls->conf.backlog = NGX_LISTEN_BACKLOG;
     ls->conf.rcvbuf = -1;
     ls->conf.sndbuf = -1;