services: Add Varnish service.

* gnu/services/web.scm (<varnish-configuration>): New record type.
(%varnish-accounts, %varnish-service-type): New variables.
(varnish-shepherd-service): New procedure.
* gnu/tests/web.scm (%varnish-vcl, %varnish-os): New variables.
(%test-varnish): New test.
* doc/guix.texi (Web Services): Document it.