service_e and auth_e. WE use different exception handlers when
invoking the AUTHENTICATE method and when invoking the service.
(do_userauth_continuation): Simplified this continuation. It is
passed a user, and returns the connection.
(do_handle_userauth): When a service is eventually invoked, it is
passed a single argument: the connection. Previously, the user
object was passed as argument to the service.
(do_userauth): Clear the user attribute of the connection, and
display a warning message if the connection was already
authenticated.
(do_userauth): Don't call make_userauth_continuation() here.

Rev: src/server_userauth.c:1.27
Rev: src/server_userauth.h:1.12