Enable optional support for user browser language

We should have a setting that says:

$form['country_code_browser_language'] = array(
  '#type' => 'radios',
  '#title' => t('Consider user browser language'),
  '#description' => t("Select this option to have the user's browser used as the language, if that language is available for the user's country."),
  '#default_value', => variable_get('country_code_browser_language', 0),
);

Then in language setting, if this variable is set, we detect the user's browser. See language_from_browser() as used in language_initialize().