Problem: When 'buftype' is "nofile" there is no overwrite check.
Solution: Also check for existing file when 'buftype' is set.
(closes vim/vim#5807)
https://github.com/vim/vim/commit/a3a9c8ef697bbf6f640cde99710b6f534f29f77d
Nvim doesn't have buftype=popup and doesn't allow buftype=terminal for
non-terminal buffer.