home: services: ssh: Allow unset boolean options in ssh-config.

From man 5 ssh_config:
Unless noted otherwise, for each parameter, the first obtained value
will be used.

We want to allow falling through to the first explicitly chosen user
defined value.

* gnu/home/services.ssh.scm (define-maybe boolean): New configuration.
(openssh-host)[forward-x11?, forward-x11-trusted?, forward-agent?,
compression?]: Replace default value with maybe-boolean.
* doc/guix.texi (Secure Shell): Update documentation to match the
changes in the code.

gnu/home/services/ssh.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2023 Janneke Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2023 Nicolas Graves <ngraves@ngraves.fr>
+;;; Copyright © 2023 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -118,6 +119,8 @@
   (string-append "  " (serialize-field-name field) " "
                  (number->string value) "\n"))
 
+(define-maybe boolean)
+
 (define (serialize-boolean field value)
   (string-append "  " (serialize-field-name field) " "
                  (if value "yes" "no") "\n"))
@@ -235,19 +238,19 @@ Additionally, the field can be left unset to allow any address family.")
    maybe-string
    "User name on the remote host.")
   (forward-x11?
-   (boolean #f)
+   maybe-boolean
    "Whether to forward remote client connections to the local X11 graphical
 display.")
   (forward-x11-trusted?
-   (boolean #f)
+   maybe-boolean
    "Whether remote X11 clients have full access to the original X11 graphical
 display.")
   (forward-agent?
-   (boolean #f)
+   maybe-boolean
    "Whether the authentication agent (if any) is forwarded to the remote
 machine.")
   (compression?
-   (boolean #f)
+   maybe-boolean
    "Whether to compress data in transit.")
   (proxy-command
    maybe-string