* guix/scripts/publish.scm (%options): Add "--advertise" option.
(show-help): Document it.
(service-name): New procedure,
(publish-service-type): new variable.
(run-publish-server): Add "advertise?" and "port" parameters. Use them to publish
the server using Avahi.
(guix-publish): Pass the "advertise?" option to "run-publish-server".
* gnu/services/base.scm (<guix-publish-configuration>): Add "advertise?"
field.
(guix-publish-shepherd-service): Honor it.
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