checklink: work around sshfs brain-damage (issue3636) With the follow_symlinks option, sshfs will successfully create links while claiming it encountered an I/O error. In addition, depending on the type of link, it may subsequently be impossible to delete the link via sshfs. Our existing link to '.' will cause sshfs to think the link is a directory, and thus cause unlink to give EISDIR. Links to non-existent names or circular links will cause the created link to not even be visible. Thus, we need to create a new temporary file and link to that. We'll still get a failure, but we'll be able to remove the link.