This would actually cause sql_generator to create the requested foreign keys.
- NONE would be the default, at least for now.
- SAFE would mean 'create the ones marked as safe' according to the new setting introduced in
MDL-30800. Eventually, this might become the default for new installs.
- ALL would be provided to make things easy for brave developers who want to see what will happen
Since config.php is included early, these probably need to be string literals 'none', etc. Not really PHP constants.