The APT config mangling is made in the chroot, as that's where all keys
corresponding to the configured APT repos can be found, and the APT pinning has
been setup. Note that, when this hook runs, the SquashFS has been generated
already, so modifying the content of the chroot at this point should not affect
the resulting ISO much.

Make it clear that ideally, we would use the right version of tails-perl5lib when building tails-iuk.

Oh, and the Cucumber test suite was fixed to nicely clean up loop devices
and mounts after itself :)

As started with efe9ecf.

Note that Math does not have its own menu entry and is only available from the
LibreOffice starter menu. That's why we're pointing to it.

... and accordingly update the design document and manual test suite steps.

The tails-upgrade-frontend program is run as the tails-upgrade-frontend user,
that is basically equivalent to root. Some of the available
tails-upgrade-frontend options might be dangerous. I've looked at it quickly and
didn't find anything scary, but still, it's simply not worth taking the risk of
privilege escalation, persistent root kit implementation, and so on.

Strictly speaking, this change does not really belong to
bugfix/7345-upgrade-from-iso-from-1.0-to-1.1, and could have been implemented
separately. However, this branch introduces running as root a syslinux binary
taken from the installed IUK, so it raised the flag that made me want to lock
this down a bit more.

... that is now waiting in the backports NEW queue. Likely it'll be accepted by
the time when someone running Wheezy needs to follow these steps.

Conflicts:
	wiki/src/doc/about/features.fr.po
	wiki/src/doc/first_steps/upgrade.pt.po
	wiki/src/support/known_issues.de.po
	wiki/src/support/known_issues.fr.po
	wiki/src/support/known_issues.mdwn
	wiki/src/support/known_issues.pt.po

Support running language_statistics.sh from the root of the repository too, and error out if it's not run from a supported place.