services: Add knot-resolver-service-type.

* gnu/services/dns.scm (<knot-resolver-configuration>): New record type.
(knot-resolver-activation, knot-resolver-shpherd-services): New procedures.
(%knot-resolver-accounts, %kresd.conf, knot-resolver-service-type): New
variables.
* doc/guix.texi (DNS Services): Document it.