services: dhcp-client: Deprecate 'dhcp-client-service' procedure.

* gnu/services/networking.scm (dhcp-client-service-type): Add default
value.
* gnu/system/examples/bare-bones.tmpl: Use (service
dhcp-client-service-type) instead of (dhcp-client-service).
* gnu/system/examples/beaglebone-black.tmpl: Likewise.
* gnu/tests/base.scm (%avahi-os): Likewise.
* gnu/tests/databases.scm (%memcached-os): Likewise.
(%mongodb-os): Likewise.
* gnu/tests/dict.scm (%dicod-os): Likewise.
* gnu/tests/mail.scm (%opensmtpd-os): Likewise.
(%exim-os): Likewise.
(%dovecot-os): Likewise.
* gnu/tests/messaging.scm (run-xmpp-test): Likewise.
(run-bitlbee-test): Likewise.
* gnu/tests/monitoring.scm (%prometheus-node-exporter-os): Likewise.
* gnu/tests/networking.scm (%inetd-os): Likewise.
(run-iptables-test): Likewise.
* gnu/tests/nfs.scm (%base-os): Likewise.
* gnu/tests/rsync.scm (%rsync-os): Likewise.
* gnu/tests/ssh.scm (run-ssh-test): Likewise.
* gnu/tests/version-control.scm (%cgit-os): Likewise.
(%git-http-os): Likewise.
(%gitolite-os): Likewise.
* gnu/tests/virtualization.scm (%libvirt-os): Likewise.
* gnu/tests/web.scm (%httpd-os): Likewise.
(%nginx-os): Likewise.
(%varnish-os): Likewise.
(%php-fpm-os): Likewise.
(%hpcguix-web-os): Likewise.
(%tailon-os): Likewise.
* tests/guix-system.sh: Likewise.
* doc/guix.texi (Networking Services): Document
'dhcp-client-service-type' and remove 'dhcp-client-service'.

gnu/tests/web.scm

@@ -125,7 +125,7 @@ HTTP-PORT."
 
 (define %httpd-os
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    (service httpd-service-type
             (httpd-configuration
              (config
@@ -154,7 +154,7 @@ HTTP-PORT."
 (define %nginx-os
   ;; Operating system under test.
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    (service nginx-service-type
             (nginx-configuration
              (log-directory "/var/log/nginx")
@@ -188,7 +188,7 @@ sub vcl_synth {
 
 (define %varnish-os
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    ;; Pretend to be a web server that serves %index.html-contents.
    (service varnish-service-type
             (varnish-configuration
@@ -237,7 +237,7 @@ echo(\"Computed by php:\".((string)(2+3)));
 (define %php-fpm-os
   ;; Operating system under test.
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    (service php-fpm-service-type)
    (service nginx-service-type
             (nginx-configuration
@@ -392,7 +392,7 @@ HTTP-PORT, along with php-fpm."
 
 (define %hpcguix-web-os
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    (service hpcguix-web-service-type
             (hpcguix-web-configuration
              (specs %hpcguix-web-specs)))))
@@ -407,7 +407,7 @@ HTTP-PORT, along with php-fpm."
 (define %tailon-os
   ;; Operating system under test.
   (simple-operating-system
-   (dhcp-client-service)
+   (service dhcp-client-service-type)
    (service tailon-service-type
             (tailon-configuration
              (config-file