Use existing setting but with new values

Use existing setting but with new values

Instead of adding another setting and having to explain how the new one
and the existing `requireClientCertificates` setting are interdependent,
let's use the existing setting and add new values.

It is changed from a boolean to a string, with the values `required`,
`optional` and `disabled`. To keep backward compatibility with the old
values, the `true` value is mapped to `required` and the `false` value
is mapped to `optional`.