mon: only all ms_handle_authentication() if auth method says we're done

mon: only all ms_handle_authentication() if auth method says we're done

Previously we would call ms_handle_authentication() possibly multiple
times, and without knowning whether it might succeed. Instead, only call
it when start_session() or handle_request() returns >0 to indicate that
we should.

Signed-off-by: Sage Weil <sage@redhat.com>