Reintroduce tails-restricted-network-detector

Originally created by @intrigeri on #10560 (Redmine)

It’s been broken at least since Tails was based on Wheezy. Some of the breakage was fixed in Tails 1.6, but even so it was still broken and the logic on which it is based does not work anymore on Jessie (#8328 (comment 93873)).

Given this history, reintroducing this mechanism should come with at least minimal automatic tests.

Related issues

• Related to #8328 (closed)