gnu: Move /etc/resolv.conf handling to the networking service.

* gnu/system/vm.scm (etc-directory): Remove 'resolv.conf'.
  (system-qemu-image): Use 'static-networking-service' with
  #:name-servers.  Don't populate the image with /etc/resolv.conf.
* gnu/system/dmd.scm (static-networking-service): Add #:name-servers
  parameter.  Use it to populate /etc/resolv.conf.