Support for RFC 3206

* pop3d/pop3d.h (ERR_SYS_LOGIN): New error code.
* pop3d/capa.c (pop3d_session_init): Advertise the AUTH-RESP-CODE
capability.
* pop3d/cmd.c (error_table) <ERR_BAD_LOGIN>: Mark with the
AUTH response code.
<ERR_SYS_LOGIN>: New entry.
* pop3d/user.c (pop3d_user): Use mu_get_auth to obtain
the user's identity. On error, the error code is ERR_BAD_LOGIN
or ERR_SYS_LOGIN, depending on its return and return from
mu_authenticate.