... 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.

This in turn allows Tails Installer to use this binary and MBR, which is
critical for avoiding problems (such as #7345) on "Upgrade from ISO".

That's what users do, and is buggy. If one already has an up-to-date Tails USB
stick, they can "Clone and upgrade", and won't bother using an ISO image.

Still, let's keep the "Clone from ISO while running the new version" test,
as it tests the Installer in the tested (new) version of Tails, which makes
little sense wrt. what users will do when the new version is out, but may catch
bugs that will affect upgrades to N+1.