Local file overwrite via predictable stderr path

Symlink following in socat before 1.8.0.2 allows local users to overwrite files via the readline.sh example script. The script redirects standard error to predictable /tmp/$USER/stderr2 after creating only the parent directory, so a preplaced symbolic link makes the shell truncate the link target when the wrapper runs. Exploitation requires local access to create the link and a victim to run the example with permission to write the targeted file; the socat executable itself is not affected.