Being able to edit a file does not mean you can write in its parent directory

Interactive `sed` needs write permission on the file itself. However, it
may create a backup file, which leads to the fact that the directory
where the file is located must be writable as well. That is a side
effect because this directory does not need to be writable.

This patch fixes this side effect by editing the file in `/tmp`, and
replaces it right after.