Router: grouped app and share fields in nxt_http_action_t.

This is a prerequisite for further introduction of openat2() features.
No functional changes.

src/nxt_http_request.c

@@ -348,9 +348,9 @@ nxt_http_application_handler(nxt_task_t *task, nxt_http_request_t *r,
         nxt_str_set(&r->server_name, "localhost");
     }
 
-    r->app_target = action->target;
+    r->app_target = action->u.app.target;
 
-    nxt_router_process_http_request(task, r, action->u.application);
+    nxt_router_process_http_request(task, r, action->u.app.application);
 
     return NULL;
 }