mesh-reads: read a node's files and accounts as an account that holds no secret (hq ADR 0310, issue 418)

Reading a file, searching one, listing a directory, the owner and mode of each
component of a path and looking up an account went through the login shell's
execute as the operator's account, which sees the operator's keys and the
agent's credentials. This module holds node-reads on every node and runs as
mesh-reads (no login, no group beside its own). It refuses the node's secret
places by name, judging the file actually opened rather than the name asked,
so a flipped link reaches nothing refused; withholds any line shaped like a
secret through mesh-sdk's outward rule and every line of a BEGIN/END block;
and bounds every answer. Replaces mesh-catalog #244 (reviews 55), whose first
commit carried a built binary.

modules/mesh-reads/module.json

@@ -0,0 +1,54 @@
+{
+  "module": "mesh-reads",
+  "version": "1",
+  "runs-as": "mesh-reads",
+  "claims": [
+    {
+      "name": "node-reads",
+      "scope": "node",
+      "serves": [
+        "stat",
+        "readlink",
+        "list",
+        "read",
+        "search",
+        "account"
+      ]
+    }
+  ],
+  "own-secrets": {
+    "broker": "${dir:state}/broker"
+  },
+  "secrets-owner": "mesh-reads",
+  "resources": [
+    {
+      "id": "account",
+      "type": "user",
+      "name": "mesh-reads",
+      "shell": "/usr/bin/nologin",
+      "home": "/var/lib/mesh-reads"
+    },
+    {
+      "id": "state",
+      "type": "directory",
+      "mode": "0700",
+      "owner": "mesh-reads",
+      "place": "."
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "tools",
+        "kind": "bundle",
+        "language": "go",
+        "system": "arch",
+        "from": "cmd/mesh-reads",
+        "binary": "mesh-reads",
+        "loads": [
+          "mesh-reads"
+        ]
+      }
+    ]
+  }
+}