guix home: Implement the 'extension-graph' and 'shepherd-graph' actions.

guix home: Implement the 'extension-graph' and 'shepherd-graph' actions.

Until now these two actions were silently ignored.

* guix/scripts/home.scm (show-help, %options): Add "--graph-backend".
(%default-options): Add 'graph-backend' key.
(export-extension-graph, export-shepherd-graph): New procedures.
(perform-action): Add #:graph-backend parameter.  Add cases for the
'extension-graph' and 'shepherd-graph' actions.
(process-action): Pass #:graph-backend to 'perform-action'.
* guix/scripts/system.scm (service-node-type)
(shepherd-service-node-type): Export
* tests/guix-home.sh: Add tests.
* doc/guix.texi (Invoking guix home): Document it.