please review'n'merge bugfix/8661-acng-expiration into testing and
devel. The corresponding change has been pushed to apt-proxy.lizard
already. This should avoid re-downloading the Tor Browser tarballs
regularly (in cases when your acng daily cleanup cronjob is working,
otherwise it won't ever delete anything I think).
Note that anyone who's using Vagrant may need to either re-provision
their basebox (or whatever it's called), or manually apply the change
to their acng.conf in their build VM.
