Serve unlink-dangling, so a leftover enable link goes through the seat

systemctl disable leaves a link whose unit file is gone, and the manager
keeps asking for the unit. The verb removes only such links in the
account's or the machine's own configuration, on the person's word, and
records each with its reason before removing it (hq issue 332).

modules/systemd/module.json

@@ -20,7 +20,8 @@
         "journal",
         "failed",
         "reset-failed",
-        "wanted-by"
+        "wanted-by",
+        "unlink-dangling"
       ]
     }
   ],
@@ -35,7 +36,10 @@
         "binary": "systemd-tools",
         "loads": [
           "systemd-tools"
-        ]
+        ],
+        "env": {
+          "MESH_SYSTEMD_STATE": "${dir:state}"
+        }
       }
     ]
   },
@@ -44,6 +48,13 @@
       "id": "package",
       "type": "package",
       "package": "systemd"
+    },
+    {
+      "id": "state",
+      "type": "directory",
+      "mode": "0700",
+      "owner": "${machine:account}",
+      "place": "."
     }
   ]
 }