- When regex type is redefined, reload it.
- New functions `foreign-regexp/case-fold-available-p', `foreign-regexp/ext-regexp-available-p'
  and `foreign-regexp/case-fold-available-p'.

- When a foreign regexp does not support some search option,
  set empty string to each search option indicator string.

- Raise error on switching search options when it's not supported by
  current foreign regexp.

- Update menu items for each search options properly
  when they are not supported.