Comment 8 for bug 1612440

An example of how to do this is in the lxd snap:

```
    sockets:
      unix:
        listen-stream: $SNAP_COMMON/lxd/unix.socket
        socket-mode: 0660
```

So sockets should go to a map of <socket-name> to objects which specify the listen-stream and (optionally) the socket-mode.