* gnu/services/herd.scm (<live-service>): New record type.
(current-services): Change to return a single value: #f or a list of
<live-service>.
* guix/scripts/system.scm (call-with-service-upgrade-info): Adjust
accordingly.
* gnu/tests/base.scm (run-basic-test)["shepherd services"]: Adjust
accordingly.