Use the BSD license consistently

The t/preamble file incorrectly mentioned the MIT license before.
While at it, use SPDX license identification headers.

Closes #149

ngx_http_fancyindex_module.c

@@ -16,7 +16,7 @@
  * Base functionality heavy based upon the stock nginx autoindex module,
  * which in turn was made by Igor Sysoev, like the majority of nginx.
  *
- * Distributed under terms of the BSD license.
+ * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <ngx_config.h>

t/preamble

@@ -3,7 +3,7 @@
 # preamble
 # Copyright (C) 2016 Adrian Perez <aperez@igalia.com>
 #
-# Distributed under terms of the MIT license.
+# SPDX-License-Identifier: BSD-2-Clause
 #
 
 function nginx_conf_generate () {