Add user_subject_as_username option

This adds an alternative option to use_identity_as_username.
In contrast to use_identity_as_username this option uses the full x509 subject as the username.
The subject is formatted as an rfc4514 distingushed name e.g. CN=client,OU=unit,O=org

Signed-off-by: Fabian Ruff <fabian@progra.de>