Update gettext-tools, libgettext8 & libiconv, remove libgettext3, debianutils, sensible-utils, some fixes.

This is part of PR #79 (which in turn is a heavily refactored version of PR #66), which in turn is about upgrading us to dpkg 1.16.

This PR here is a lot smaller and simpler, and we should in principle be able to apply right away. That would make it a lot easier to eventually merge #79.

Note that if we merge this, then at the same time, we should probably remove the "essential" flag from debianutils and sensible-utils in the dists trees. Hmm, I vaguely recall that one should be careful about switching a package from essential to unessential, though... but the only reason for that I can think of right now is that some package might have a hidden dep on these packages, which would become only apparent if somebody removed those packages. But perhaps I am forgetting something, so it would be nice if some other folks could review this.

Note that the code here passes "make test" and bootstraps fine. I have not yet tested injecting it, though.