Assistant_uses_obsolete_GDU_volume_monitor.mdwn

### What steps will reproduce the problem?

Run `git annex assistant`.


### What is the expected output? What do you see instead?

git-annex complains:

     dbus failed; falling back to mtab polling (ClientError {clientErrorMessage =
        "Call failed: The name org.gtk.Private.GduVolumeMonitor was not provided
        by any .service files", clientErrorFatal = False})

This is because the `gvfs-gdu-volume-monitor` daemon has been obsoleted and removed from GNOME 3.6 (maybe even earlier).

git-annex should start using `gvfs-udisks2-volume-monitor` at bus name `org.gtk.Private.UDisks2VolumeMonitor`.

Alternatively, git-annex should stop relying on any per-user services, and use kernel interfaces directly when available. (This way, monitoring could work even if the user wasn't logged in and/or didn't have a DBus session bus.)

  * On all Linux kernels since 2.6.15, the `/proc/self/mounts` file is pollable – you can use **select(), poll() or epoll** to detect new mounted filesystems, without having to rely on periodic checks. (Run `findmnt -p` to see it in action.)

  * On BSD systems, kqueue on `/etc/mtab`.

### What version of git-annex are you using? On what operating system?

git-annex 3.20130102 on Linux 3.7.1, GNOME 3.7