Moving request limit control to libunit.

Introducting application graceful stop.  For now only used when application
process reach request limit value.

This closes #585 issue on GitHub.

src/nodejs/unit-http/unit.h

@@ -41,7 +41,8 @@ private:
     void shm_ack_handler(nxt_unit_ctx_t *ctx);
 
     static int add_port(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port);
-    static void remove_port(nxt_unit_t *unit, nxt_unit_port_t *port);
+    static void remove_port(nxt_unit_t *unit, nxt_unit_ctx_t *ctx,
+                            nxt_unit_port_t *port);
 
     static void quit_cb(nxt_unit_ctx_t *ctx);
     void quit(nxt_unit_ctx_t *ctx);