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.

1 files changed, 2 insertions, 1 deletions

diff --git a/doc.am b/doc.am
index 71a65ba0c47..1980cc870e8 100644
--- a/doc.am
+++ b/doc.am
@@ -23,7 +23,8 @@ DOT_FILES =					\
   doc/images/bootstrap-graph.dot                \
   doc/images/coreutils-graph.dot                \
   doc/images/coreutils-bag-graph.dot            \
-  doc/images/service-graph.dot
+  doc/images/service-graph.dot                  \
+  doc/images/dmd-graph.dot
 
 DOT_VECTOR_GRAPHICS =                           \
   $(DOT_FILES:%.dot=%.eps)                      \