* guix/graph.scm (%graph-backends): New variable.
* guix/scripts/graph.scm (lookup-backend, list-backends): New
procedures.
(%options): Add options for "backend" and "list-backends".
(show-help): Add help texts for "backend" and "list-backend" options.
(%default-options): Add "backend" default.
(guix-graph): Pass backend argument to "export-graph".
* doc/guix.texi (Invoking guix graph): Document the new options.
* guix/graph.scm (<graph-backend>): Add fields "name" and "description".
(%graphviz-backend): Provide values for name and description.
(export-graph): Ignore name and description when matching backends.
(graph-backend-name, graph-backend-description): New procedures.
* tests/graph.scm (make-recording-backend): Initialize name and
description fields of test graph-backend.
* guix/graph.scm (traverse/depth-first): New procedure, based on code
formerly in 'node-transitive-edges'.
(node-transitive-edges): Rewrite in terms of it.
