Fixing idle connection close function.

There was a typo: nxt_queue_head() used instead of nxt_queue_first() in
connection iteration loop.  This prevents idle connection close on quit.

This closes #334 issue on GitHub.
Thanks to 洪志道 (Hong Zhi Dao).

src/nxt_runtime.c

@@ -459,7 +459,7 @@ nxt_runtime_close_idle_connections(nxt_event_engine_t *engine)
 
     idle = &engine->idle_connections;
 
-    for (link = nxt_queue_head(idle);
+    for (link = nxt_queue_first(idle);
          link != nxt_queue_tail(idle);
          link = next)
     {