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/plex/module.json

@@ -27,22 +27,27 @@
   ],
   "accesses": [
     {
+      "id": "movies",
       "path": "/services/media/movies",
       "mode": "read"
     },
     {
+      "id": "series",
       "path": "/services/media/series",
       "mode": "read"
     },
     {
+      "id": "anime",
       "path": "/services/media/anime",
       "mode": "read"
     },
     {
+      "id": "music",
       "path": "/services/media/music",
       "mode": "read"
     },
     {
+      "id": "audiobooks",
       "path": "/services/media/audiobooks",
       "mode": "read"
     }
@@ -82,11 +87,11 @@
       "volumes": [
         "/services/plex/config:/config",
         "/services/plex/transcode:/transcode",
-        "/services/media/movies:/movies",
-        "/services/media/series:/series",
-        "/services/media/anime:/anime",
-        "/services/media/music:/music",
-        "/services/media/audiobooks:/audiobooks"
+        "${access:movies}:/movies",
+        "${access:series}:/series",
+        "${access:anime}:/anime",
+        "${access:music}:/music",
+        "${access:audiobooks}:/audiobooks"
       ]
     },
     {