Enable discovering of real user DN to break loose from the uid=%s,%s pattern.

Most LDAP directories are not flat and the RDNs are not uid=*.

This feature update introduces a config option discover_dn that, after searching the directory for the user object, uses the discovered DN for the authentication bind.

Leaving discover_dn false keeps the old behaviour.

This enables the code to work with arbitrarily complex directories and has been verified to work.