services: xorg: Enable override of xserver-arguments.

* gnu/services/xorg.scm (xorg-start-command): Add parameter #:xserver-arguments.
* doc/guix.texi (X Window): Document it.

doc/guix.texi

@@ -31,7 +31,7 @@ Copyright @copyright{} 2016, 2017, 2018 Chris Marusich@*
 Copyright @copyright{} 2016, 2017, 2018 Efraim Flashner@*
 Copyright @copyright{} 2016 John Darrington@*
 Copyright @copyright{} 2016, 2017 Nils Gillmann@*
-Copyright @copyright{} 2016, 2017, 2018 Jan Nieuwenhuizen@*
+Copyright @copyright{} 2016, 2017, 2018, 2019 Jan Nieuwenhuizen@*
 Copyright @copyright{} 2016 Julien Lepiller@*
 Copyright @copyright{} 2016 Alex ter Weele@*
 Copyright @copyright{} 2016, 2017, 2018, 2019 Christopher Baines@*
@@ -13140,6 +13140,7 @@ type @code{<sddm-configuration>}.
   [#:fonts %default-xorg-fonts] @
   [#:configuration-file (xorg-configuration-file @dots{})] @
   [#:xorg-server @var{xorg-server}]
+  [#:xserver-arguments '("-nolisten" "tcp")]
 Return a @code{startx} script in which @var{modules}, a list of X module
 packages, and @var{fonts}, a list of X font directories, are available.  See
 @code{xorg-wrapper} for more details on the arguments.  The result should be
@@ -13148,6 +13149,22 @@ used in place of @code{startx}.
 Usually the X server is started by a login manager.
 @end deffn
 
+@cindex @code{-listen tcp}, for X11.
+This procedure is useful to override command line options for the X server,
+such as having it listen to over TCP:
+
+@example
+(operating-system
+  ...
+  (services
+    (modify-services %desktop-services
+      (slim-service-type config =>
+        (slim-configuration
+          (inherit config)
+          (startx (xorg-start-command
+                   #:xserver-arguments '("-listen" "tcp"))))))))
+@end example
+
 @deffn {Scheme Procedure} xorg-configuration-file @
   [#:modules %default-xorg-modules] @
   [#:fonts %default-xorg-fonts] @

gnu/services/xorg.scm

@@ -3,6 +3,7 @@
 ;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
 ;;; Copyright © 2018, 2019 Timothy Sample <samplet@ngyro.com>
+;;; Copyright © 2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -292,7 +293,8 @@ in place of @code{/usr/bin/X}."
                              (configuration-file
                               (xorg-configuration-file #:modules modules
                                                        #:fonts fonts))
-                             (xorg-server xorg-server))
+                             (xorg-server xorg-server)
+                             (xserver-arguments '("-nolisten" "tcp")))
   "Return a @code{startx} script in which @var{modules}, a list of X module
 packages, and @var{fonts}, a list of X font directories, are available.  See
 @code{xorg-wrapper} for more details on the arguments.  The result should be
@@ -305,8 +307,8 @@ used in place of @code{startx}."
   (define exp
     ;; Write a small wrapper around the X server.
     #~(apply execl #$X #$X ;; Second #$X is for argv[0].
-             "-logverbose" "-verbose" "-nolisten" "tcp" "-terminate"
+             "-logverbose" "-verbose" "-terminate" #$@xserver-arguments
-             (cdr (command-line))))
+              (cdr (command-line))))
 
   (program-file "startx" exp))