bug#37318: OpenNTPD generated config is convoluted

[Maxim Cournoyer]

For the documented following openntpd-service-type definition:

--8<---------------cut here---------------start------------->8---
(openntpd-configuration
            (listen-on '("127.0.0.1" "::1"))
            (sensor '("udcf0 correction 70000"))
            (constraint-from '("www.gnu.org"))
            (constraints-from '("https://www.google.com/";))
            (allow-large-adjustment? #t)))
--8<---------------cut here---------------end--------------->8---

The following configuration file is generated:

--8<---------------cut here---------------start------------->8---
listen on 127.0.0.1
 listen on ::1
constraints from "https://www.google.com/";
constraints from "https://www.google.com/";
sensor udcf0 correction 70000
constraints from "https://www.google.com/";
server 0.guix.pool.ntp.org
 server 1.guix.pool.ntp.org
 server 2.guix.pool.ntp.org
 server 3.guix.pool.ntp.org
constraints from "https://www.google.com/";
constraints from "https://www.google.com/";
constraint from www.gnu.org
--8<---------------cut here---------------end--------------->8---

Notice the repeated "constraints from" directives.