revert r3864, it may cause kernel panic on some kernels while working with IPv6

src/os/unix/ngx_linux_config.h

diff --git a/src/os/unix/ngx_linux_config.h b/src/os/unix/ngx_linux_config.h
index 556651c..cf45e54 100644
--- a/src/os/unix/ngx_linux_config.h
+++ b/src/os/unix/ngx_linux_config.h
@@ -88,7 +88,7 @@
 #endif
 
 
-#define NGX_LISTEN_BACKLOG        -1
+#define NGX_LISTEN_BACKLOG        511
 
 
 #if defined TCP_DEFER_ACCEPT && !defined NGX_HAVE_DEFERRED_ACCEPT