web: 'tls-wrap' returns an unbuffered custom port.

This mirrors Guix commit b168acae2a01fd84075cc134a6140594a978fde5.

* module/web/client.scm (tls-wrap)[unbuffered]: New procedure.
Pass the result of 'make-custom-binary-input/output-port' to
'unbuffered'.

module/web/client.scm

@@ -274,17 +274,24 @@ host name without trailing dot."
         (unless (port-closed? record)
           (close-port record)))
 
+      (define (unbuffered port)
+        (setvbuf port 'none)
+        port)
+
       (setvbuf record 'block)
 
       ;; Return a port that wraps RECORD to ensure that closing it also
       ;; closes PORT, the actual socket port, and its file descriptor.
+      ;; Make sure it does not introduce extra buffering (custom ports
+      ;; are buffered by default).
       ;; XXX: This wrapper would be unnecessary if GnuTLS could
       ;; automatically close SESSION's file descriptor when RECORD is
       ;; closed, but that doesn't seem to be possible currently (as of
       ;; 3.6.9).
-      (make-custom-binary-input/output-port "gnutls wrapped port" read! write!
-                                            get-position set-position!
-                                            close))))
+      (unbuffered
+       (make-custom-binary-input/output-port "gnutls wrapped port" read! write!
+                                             get-position set-position!
+                                             close)))))
 
 (define (ensure-uri-reference uri-or-string)
   (cond