GUnixMount: port unmount to GSubprocess

The existing code is buggy and now that we have GSubprocess, we should just use
it instead, allowing for some substantial reduction in complexity.

https://bugzilla.gnome.org/show_bug.cgi?id=724916
GUnixVolume: port to GSubprocess

Do the same as we just did for GUnixMount, for GUnixVolume too.

https://bugzilla.gnome.org/show_bug.cgi?id=724916
GUnixVolume: implement _finish functions

The _finish functions for GUnixVolume _mount and _eject functions were
never implemented, having been simply stubbed out as 'return TRUE;'.

Implement them.

https://bugzilla.gnome.org/show_bug.cgi?id=724916