ssh: Pass an empty "exceptfds" set to 'select'.

Previously the redirect code could end up exiting prematurely because of
an uninteresting "exceptional condition" on the socket (info "(libc)
Waiting for I/O").

* guix/ssh.scm (remote-daemon-channel): Pass the empty list as the third
argument to 'select'.  It was a mistake to pass a non-empty list there
in the first place.

guix/ssh.scm

@@ -124,8 +124,8 @@ Throw an error on failure."
          (connect sock AF_UNIX ,socket-name)
 
          (let loop ()
-           (match (select (list stdin sock) '() (list stdin stdout sock))
+           (match (select (list stdin sock) '() '())
-             ((reads writes ())
+             ((reads () ())
               (when (memq stdin reads)
                 (match (read! stdin buffer)
                   ((? zero?)                      ;EOF