dbus: hold node-message-bus, and never restart the bus live (hq ADR 0215)

A live restart of the system bus during an upgrade hung every login on a
workstation until a reboot. The module owns the bus's packages, declares
the bus running with no restart or reload trigger, publishes only curated
events (health, services, denials; never traffic) and serves tools to look
at both buses.

modules/dbus/module.json

@@ -0,0 +1,67 @@
+{
+  "module": "dbus",
+  "version": "1",
+  "capabilities": [
+    "package-manager",
+    "service-manager"
+  ],
+  "claims": [
+    {
+      "name": "node-message-bus",
+      "scope": "node"
+    }
+  ],
+  "emits": [
+    "bus.stalled",
+    "bus.recovered",
+    "bus.restarted",
+    "service.appeared",
+    "service.left",
+    "policy.denied"
+  ],
+  "tools": [
+    "dbus_names",
+    "dbus_introspect",
+    "dbus_monitor",
+    "dbus_check",
+    "dbus_health"
+  ],
+  "resources": [
+    {
+      "id": "dbus",
+      "type": "package",
+      "package": "dbus"
+    },
+    {
+      "id": "broker",
+      "type": "package",
+      "package": "dbus-broker"
+    },
+    {
+      "id": "broker-units",
+      "type": "package",
+      "package": "dbus-broker-units"
+    },
+    {
+      "id": "system-bus",
+      "type": "service",
+      "unit": "dbus-broker.service",
+      "state": "running"
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "tools-go",
+        "kind": "bundle",
+        "language": "go",
+        "system": "arch",
+        "from": "cmd/dbus-tools",
+        "binary": "dbus-tools",
+        "loads": [
+          "dbus-tools"
+        ]
+      }
+    ]
+  }
+}