Core: supported directory skipping in ngx_walk_tree().

If pre_tree_handler() returns NGX_DECLINED, the directory is ignored.
diff --git a/src/core/ngx_file.c b/src/core/ngx_file.c
index 8657d69..dc22596 100644
--- a/src/core/ngx_file.c
+++ b/src/core/ngx_file.c
@@ -1047,10 +1047,18 @@
             ctx->access = ngx_de_access(&dir);
             ctx->mtime = ngx_de_mtime(&dir);
 
-            if (ctx->pre_tree_handler(ctx, &file) == NGX_ABORT) {
+            rc = ctx->pre_tree_handler(ctx, &file);
+
+            if (rc == NGX_ABORT) {
                 goto failed;
             }
 
+            if (rc == NGX_DECLINED) {
+                ngx_log_debug1(NGX_LOG_DEBUG_CORE, ctx->log, 0,
+                               "tree skip dir \"%s\"", file.data);
+                continue;
+            }
+
             if (ngx_walk_tree(ctx, &file) == NGX_ABORT) {
                 goto failed;
             }