Make mounts refuse what it could break: data the mesh keeps, a busy or non-empty mount point, its own share, and a mount that is not its source; say each node's wants so the server exports only to them (review of #136)

modules/mounts/module.json

@@ -7,6 +7,9 @@
   "reads": [
     "nfs-server.exports"
   ],
+  "invokes": [
+    "seat:mesh-controller.data"
+  ],
   "capabilities": [
     "package-manager",
     "service-manager"
@@ -82,7 +85,7 @@
       "type": "file",
       "path": "/etc/mesh-mounts/mounts.conf",
       "mode": "0644",
-      "content": "# Written by the mesh (module mounts, novox/hq ADR 0263) from this machine's assignment: the settings\n# shares and sources, and the binding of nfs-share. Replaced on every push; change the settings, not this.\n# The module's process reads it as root every 30 seconds and writes one .mount and .automount per entry.\nshares=${setting:shares}\nsources=${setting:sources}\nserver=${bound:nfs-share:from}\nat=${bound:nfs-share:at}\naccount=${machine:account}\nsecret=${dir:state}/smb-credentials.secret\nstate=${dir:state}\n"
+      "content": "# Written by the mesh (module mounts, novox/hq ADR 0263) from this machine's assignment: the settings\n# shares and sources, and the binding of nfs-share. Replaced on every push; change the settings, not this.\n# The module's process reads it as root every 30 seconds and writes one .mount and .automount per entry.\nshares=${setting:shares}\nsources=${setting:sources}\nserver=${bound:nfs-share:from}\nat=${bound:nfs-share:at}\naccount=${machine:account}\nnode=${machine:name}\nsecret=${dir:state}/smb-credentials.secret\nstate=${dir:state}\n"
     },
     {
       "id": "bus-dir-parent",