guix system: Add 'dmd-graph' command.

* guix/scripts/system.scm (dmd-service-node-label,
  dmd-service-node-type, export-dmd-graph): New procedures.
  (show-help): Add 'dmd-graph'.
  (guix-system)[parse-sub-command]: Likewise.
  Honor it.
* doc/guix.texi (Invoking guix system): Document it.
  (dmd Services): Add an illustration and explanation.
* doc/images/dmd-graph.dot: New file.
* doc.am (DOT_FILES): Add it.