An access has an id, and its mounts name it (issue 153)

Ten definitions name each access by id; the path stays as the default an assignment may replace,
and the host side of every mount says ${access:<id>}. Resolved with no placement, every definition
names exactly the paths it named before (TestPlacedDirectoriesKeepTheirPaths, extended). On an
adopted machine the assignment now says `accesses: {<id>: <path>}` and the mount follows.

Needs the controller that knows an access id (mesh-controller #176); the running one refuses the
field at registration.

modules/bookshelf/module.json

@@ -24,10 +24,12 @@
   ],
   "accesses": [
     {
+      "id": "books",
       "path": "/services/media/books",
       "mode": "read-write"
     },
     {
+      "id": "downloads",
       "path": "/services/media/downloads",
       "mode": "read-write"
     }
@@ -61,8 +63,8 @@
       ],
       "volumes": [
         "/services/bookshelf/config:/config",
-        "/services/media/books:/books",
-        "/services/media/downloads:/downloads"
+        "${access:books}:/books",
+        "${access:downloads}:/downloads"
       ]
     },
     {