On the long run, we want to do this at build time, just like the TorBrowser.
However, fiddling with the build options two days before a Tails release seems
crazy, so let's disable this with a pref for now.

Successfully tested on http://mozilla.github.io/webrtc-landing/ and
http://net.ipcalf.com/.

In Tails 0.22, that's how it's called almost everywhere.

Rationale: make things easier for auditors.

The tails-upgrade-frontend program now runs as this user, and it needs to reach
the Internet to get an upgrade-description file.