services: guix: Add nar-herder-service-type.
* gnu/services/guix.scm (<nar-herder-configuration>): New record type.
(nar-herder-configuration, nar-herder-configuration?,
nar-herder-configuration-package,
nar-herder-configuration-user,
nar-herder-configuration-group,
nar-herder-configuration-mirror
nar-herder-configuration-database
nar-herder-configuration-database-dump
nar-herder-configuration-host
nar-herder-configuration-port
nar-herder-configuration-storage
nar-herder-configuration-storage-limit
nar-herder-configuration-storage-nar-removal-criteria
nar-herder-shepherd-services, nar-herder-activation,
nar-herder-account): New procedures.
(nar-herder-service-type): New variable.
* gnu/tests/guix.scm (%test-nar-herder): New variable.
* doc/guix.texi (Guix Services): Document the new service.
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