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.

module.json

@@ -89,6 +89,7 @@
     "cleanup",
     "dead-letters",
     "root-free",
+    "judge-directories",
     "data",
     "build",
     "artifacts",