Ask a node's engine which of its directories the walk would refuse, read-only (issue 358)

Before the node-engine gives a mesh directory its mode and owner only through
a walk that follows no link another account could make, the operator is to
see which directories that walk would refuse on every node, the control-node
included, where no tool reads a path. `node judge-directories <node>` signs
the ask as a hand-over is, under its own context, and prints the engine's
one line per directory; it changes nothing, so it is a seat verb an agent may
call. The node is granted the subject, the controller alone its publish.

cmd/mesh-controller/status_summary.go

@@ -172,7 +172,7 @@ func composeStatus(open *stores) func(context.Context) ([]byte, error) {
 var readingVerbs = map[string]bool{
 	"tools": true, "calls": true, "status": true, "nodes": true, "node": true, "modules": true,
 	"seats": true, "builds": true, "plan": true, "queue": true, "durations": true, "hand-acts": true,
-	"doctor": true, "conditions": true, "healers": true,
+	"doctor": true, "conditions": true, "healers": true, "judge-directories": true,
 }
 
 // nudgingListener is the enrolment, nudging the summary when a machine said something new.