Configuration persistence.

Now configuration survives server reloads.

src/nxt_runtime.h

@@ -58,6 +58,9 @@ struct nxt_runtime_s {
     const char             *pid;
     const char             *log;
     const char             *modules;
+    const char             *state;
+    const char             *conf;
+    const char             *conf_tmp;
     const char             *control;
 
     nxt_queue_t            engines;            /* of nxt_event_engine_t */