services: Add dhcpd-service-type and <dhcpd-configuration>.

services: Add dhcpd-service-type and <dhcpd-configuration>.

* doc/guix.texi (Networking Services): Document it.
* gnu/services/networking.scm (dhcpd-service-type): Add it.
(dhcpd-configuration, dhcpd-configuration?): Add it.
(dhcpd-configuration-package): Add it.
(dhcpd-configuration-config-file): Add it.
(dhcpd-configuration-version): Add it.
(dhcpd-configuration-run-directory): Add it.
(dhcpd-configuration-lease-file): Add it.
(dhcpd-configuration-pid-file): Add it.
(dhcpd-configuration-interfaces): Add it.
* gnu/tests/networking.scm (minimal-dhcpd-v4-config-file)
(dhcpd-v4-configuration, %dhcpd-os, run-dhcpd-test, %test-dhcpd): New
variables.