Every time ltspimage update status changes, trigger a script
at /usr/lib/puavo-ltsp-install/update-ltspimage-update-stats.
This is going to do some dbus-calls (I think) to update the state
in puavo-client-updater-applet.
${puavoinstall_libdir}/update-ltspimage-update-stats is allowed to fail,
and we are not interested in its error messages.
Direct stdout as well to /dev/null for cmp.
Pass information about image update progress to
puavo-client-updater-applet.  It does not show it yet (except
printing to stdout), but at least it gets it.