Renamed NGX_THREADS to NGX_OLD_THREADS because of deprecation.

It's mostly dead code and the original idea of worker threads has been rejected.
diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
index d69783f..11e413f 100644
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -26,7 +26,7 @@
 ngx_uint_t             ngx_test_config;
 ngx_uint_t             ngx_quiet_mode;
 
-#if (NGX_THREADS)
+#if (NGX_OLD_THREADS)
 ngx_tls_key_t          ngx_core_tls_key;
 #endif