new: [0km] check for existence of ``rrdtool`` or hint for a way to install it

bin/0km

@@ -1337,6 +1337,15 @@ cmdline.spec::cmd:vps-stats:run() {
         echo ""
         return 1
     } >&2
+    ( depends rrdtool ) || {
+        echo ""
+        echo "  Rrdtool is required to display graphs..." \
+             "You might want to try to install ${WHITE}rrdtool${NORMAL} with:"
+        echo ""
+        echo "    apt install rrdtool"
+        echo ""
+        return 1
+    } >&2
 
     export GNUTERM=qt
     exec {PFD}> >(exec gnuplot 2>/dev/null)