services: dicod: Add 'interfaces' configuration field.

This makes 'dicod' listen on 'localhost' by default, whereas it was
previously listening on all the interfaces, which is not a good default
security-wise.

* gnu/services/dict.scm (<dicod-configuration>)[interfaces]: New field.
(dicod-configuration-file)[database->text]: New procedure, with code
formerly in 'dicod-configuration->text'.
[dicod-configuration->text]: Rename to...
[configuration->text]: ... this.  Honor 'interfaces'.
* doc/guix.texi (Various Services): Document 'interfaces'.