Replace deprecated create_function() with Closures

create_function() has been deprecated as of PHP 7.2

Occurrences of create_function() in /plugin.php were intentionally
excluded from this update as that source code is run when PHP < 5.4 is
detected (Closures require PHP 5.4+).

See wpsharks/comet-cache#921

src/includes/traits/Shared/FsUtils.php

@@ -30,7 +30,7 @@ public function nDirSeps($dir_file, $allow_trailing_slash = false)
         }
         if (mb_strpos($dir_file, ':' !== false)) {
             if (preg_match('/^(?P<drive_letter>[a-zA-Z])\:[\/\\\\]/u', $dir_file)) {
-                $dir_file = preg_replace_callback('/^(?P<drive_letter>[a-zA-Z])\:[\/\\\\]/u', create_function('$m', 'return mb_strtoupper($m[0]);'), $dir_file);
+                $dir_file = preg_replace_callback('/^(?P<drive_letter>[a-zA-Z])\:[\/\\\\]/u', function ($m) { return mb_strtoupper($m[0]); }, $dir_file);
             }
         }
         $dir_file = preg_replace('/\/+/u', '/', str_replace([DIRECTORY_SEPARATOR, '\\', '/'], '/', $dir_file));