ftp-client: Add missing CR in "USER" command.

* guix/ftp-client.scm (%ftp-login): Add #\return before #\newline.
  Fixes access to some FTP servers, such as
  ftp://invisible-island.net ("ProFTPD 1.3.4a Server").

guix/ftp-client.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2010, 2011, 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2010, 2011, 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -73,7 +73,8 @@ (define (%ftp-command command expected-code port)
         (throw 'ftp-error port command code message))))
 
 (define (%ftp-login user pass port)
-  (let ((command (string-append "USER " user (string #\newline))))
+  (let ((command (string-append "USER " user
+                                (string #\return) (string #\newline))))
     (display command port)
     (let-values (((code message) (%ftp-listen port)))
       (case code