We could also overhaul the code of the openntpd-service-type so that it'd
be possible to provide different server types and options like for the
'ntp-service-type', but I'm not sure it's worth it, given that OpenNTPD
is more spartan than NTP (it only supports two server types ('server' vs
'servers'), already captured as fields in its configuration record; and
its server directives only support one option, 'weight', which can
currently be included in the server string if desired).

However, I noticed that the configuration file produced by the openntpd
service, while valid, is not very clean.

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/"
constraints from "https://www.google.com/"
servers 0.guix.pool.ntp.org
constraints from "https://www.google.com/"
constraint from www.gnu.org
--8<---------------cut here---------------end--------------->8---

This is reproducible when testing without my changes (with the difference
that multiple servers were used instead of a single "pool" entry point).

I've opened an issue so that this issue can be tracked separately here:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=37318.

Thank you!

Maxim