Implemented "return" action.

The "return" action can be used to immediately generate a simple HTTP response
with an arbitrary status:

  {
      "action": {
          "return": 404
      }
  }

This is especially useful for denying access to specific resources.

src/nxt_http_return.c

@@ -0,0 +1,42 @@
+
+/*
+ * Copyright (C) NGINX, Inc.
+ */
+
+#include <nxt_router.h>
+#include <nxt_http.h>
+
+
+static const nxt_http_request_state_t  nxt_http_return_send_state;
+
+
+nxt_http_action_t *
+nxt_http_return_handler(nxt_task_t *task, nxt_http_request_t *r,
+    nxt_http_action_t *action)
+{
+    nxt_http_status_t  status;
+
+    status = action->u.return_code;
+
+    if (status >= NXT_HTTP_BAD_REQUEST
+        && status <= NXT_HTTP_SERVER_ERROR_MAX)
+    {
+        nxt_http_request_error(task, r, status);
+        return NULL;
+    }
+
+    r->status = status;
+    r->resp.content_length_n = 0;
+    r->state = &nxt_http_return_send_state;
+
+    nxt_http_request_header_send(task, r, NULL, NULL);
+
+    return NULL;
+}
+
+
+static const nxt_http_request_state_t  nxt_http_return_send_state
+    nxt_aligned(64) =
+{
+    .error_handler = nxt_http_request_error_handler,
+};