Can't use symlink for wp-cache-config.php since 1.6.5

I use a deployment tool to deploy my app, and in order to keep the same config when deploying a new release, use a symbolic link from `wp-content/wp-cache-config.php` to my permanent shared config file, stored, say, in a $SHARED directory (`ln -s $SHARED/wp-cache-config.php $RELEASE/wp-content/wp-cache-config.php`).

- Expected behaviour : modifying the configuration via the advanced options page should, and did until 1.6.4, modify the `$SHARED/wp-cache-config.php` file, thanks to the symlink.

- Bug description : since version 1.6.5, the symlink is deleted and overwritten with a new config file, which results in losing configuration at each deployment.

Related to the Automattic/wp-super-cache#652 fix maybe ?

Start by tracing how the advanced options page writes wp-content/wp-cache-config.php, comparing the behavior with version 1.6.4 and reviewing the related Automattic/wp-super-cache#652 fix. Done means updating the configuration through the advanced options page preserves the symlink and modifies the shared target file instead of replacing the link.

Written by the indexing model from the issue text.