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'.