processes: Write to $PAGER.

* guix/ui.scm (with-paginated-output-port): Export.
* guix/scripts/processes.scm (guix-processes): Use it instead of writing
directly to (current-output-port).

guix/scripts/processes.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -231,7 +231,8 @@ (define options
                 cons
                 '()))
 
-  (for-each (lambda (session)
-              (daemon-session->recutils session (current-output-port))
-              (newline))
-            (daemon-sessions)))
+  (with-paginated-output-port port
+    (for-each (lambda (session)
+                (daemon-session->recutils session port)
+                (newline port))
+              (daemon-sessions))))

guix/ui.scm

@@ -121,6 +121,7 @@ (define-module (guix ui)
             file-hyperlink
             location->hyperlink
 
+            with-paginated-output-port
             relevance
             package-relevance
             display-search-results