gnu: guile-ssh: Close RREPL channel ports before they are finalized.

Partly fixes <https://bugs.gnu.org/26976>.

* gnu/packages/patches/guile-ssh-channel-finalization.patch: New file.
* gnu/packages/ssh.scm (guile-ssh)[source](patches): Use it.
* gnu/local.mk (dist_patch_DATA): Add it.

gnu/packages/patches/guile-ssh-channel-finalization.patch

@@ -0,0 +1,28 @@
+Avoid asynchronous channel finalization, which could lead to segfaults due to
+libssh not being thread-safe: <https://bugs.gnu.org/26976>.
+
+--- guile-ssh-0.11.0/modules/ssh/dist/node.scm	2017-06-13 14:37:44.861671297 +0200
++++ guile-ssh-0.11.0/modules/ssh/dist/node.scm	2017-06-13 14:38:02.841580565 +0200
+@@ -391,11 +391,18 @@ listens on an expected port, return #f o
+   "Evaluate QUOTED-EXP on the node and return the evaluated result."
+   (let ((repl-channel (node-open-rrepl node)))
+     (rrepl-skip-to-prompt repl-channel)
+-    (call-with-values (lambda () (rrepl-eval repl-channel quoted-exp))
+-      (lambda vals
+-        (and (node-stop-repl-server? node)
++    (dynamic-wind
++      (const #t)
++      (lambda ()
++        (rrepl-eval repl-channel quoted-exp))
++      (lambda ()
++        (when (node-stop-repl-server? node)
+              (node-stop-server node))
+-        (apply values vals)))))
++
++        ;; Close REPL-CHANNEL right away to prevent finalization from
++        ;; happening in another thread at the wrong time (see
++        ;; <https://bugs.gnu.org/26976>.)
++        (close-port repl-channel)))))
+ 
+ (define (node-eval-1 node quoted-exp)
+   "Evaluate QUOTED-EXP on the node and return the evaluated result.  The