forbid insecure repositories by default expect in apt-get

With this commit all APT-based clients default to refusing to work with
unsigned or otherwise insufficently secured repositories. In terms of
apt and apt-get this changes nothing, but it effects all tools using
libapt like aptitude, synaptic or packagekit.

The exception remains apt-get for stretch for now as this might break
too many scripts/usecases too quickly.

The documentation is updated and extended to reflect how to opt out or
in on this behaviour change.

Closes: 808367