Mail: fixed EXTERNAL to be accepted only if enabled. As originally implemented in 6774:bcb107bb89cd, it wasn't possible to disable the EXTERNAL authentication method: it was always accepted (but not advertised unless enabled). It is, however, believed that it is better to reject attempts to use the disabled method, hence in 6869:b2915d99ee8d an attempt was made to address this. This attempt was insufficient though: it was still possible to use the method as long as initial SASL response was used. With this patch both challenge-response and initial response forms are disabled. Additionally, initial response handling for the PLAIN authentication is removed from ngx_mail_auth_parse(), for consistency and to don't provoke such bugs.