ftp-client: Restrict to TCP/IP connections.

Fixes <http://bugs.gnu.org/21925>.
Regression introduced in 279ec1d.
Reported by Chris Marusich <cmmarusich@gmail.com>.

* guix/ftp-client.scm (ftp-open): Restrict sockets to
  SOCK_STREAM/IPPROTO_IP.

guix/ftp-client.scm

@@ -140,8 +140,9 @@ TIMEOUT, an ETIMEDOUT error is raised."
 
   (let loop ((addresses addresses))
     (let* ((ai (car addresses))
-           (s  (socket (addrinfo:fam ai) SOCK_STREAM ;TCP only
-                       (addrinfo:protocol ai))))
+           (s  (socket (addrinfo:fam ai)
+                       ;; TCP/IP only
+                       SOCK_STREAM IPPROTO_IP)))
 
       (catch 'system-error
         (lambda ()