Bug#212921: cdebconf: Merge in new translations in templates

[Petter Reinholdtsen <pere@hungry.com>]

[Denis Barbier]
> Ack.

Very good.  This make it possible to add extra translations by adding
packages to the CD and listing them in the udeb_include file. :)

Now we just need to start using this feature. :)

> The current behaviour should certainly still be the default, so I
> added a --merge command-line flag to debconf-loadtemplate.

I'm unable to come up with any sensible scenario where one want to
keep the current behaviour.

If the database contain foo and foo-ll, and then is loaed with
identical foo and only foo-nl, I see no reason to throw away foo-ll.
Do you?