Replace hardcoded configuration path '/config.php' with CONFIG_FILE

[maestrow]

Replace hardcoded configuration path '/config.php' with CONFIG_FILE named constant. Thus you can embed file manager with different configs.

  define('FM_EMBED', true);
  define('CONFIG_FILE', __DIR__.'/config1.php');  // this value can depends on some state
  print(CONFIG_FILE);
  define('FM_SELF_URL', $_SERVER['PHP_SELF']);
  require './tinyfilemanager.php';

This PR made possible #444 feature request: "allow user to access multiple directories". Use this scenario: For each target directory define a link which lead to it's own page with embedded file manager instance, having its own CONFIG_FILE. Common configuration options can be included from shared file.

[fjmdevs]

Disregard my accidental click 🤷‍♂️