support lang= and target= sources.list options

support lang= and target= sources.list options

We support arch= for a while, now we finally add lang= as well and as a
first simple way of controlling which targets to acquire also target=.

This asked for a redesign of the internal API of parsing and storing
information about 'deb' and 'deb-src' lines. As this API isn't visible
to the outside no damage done through.

Beside being a nice cleanup (= it actually does more in less lines) it
also provides us with a predictable order of architectures as provides
in the configuration rather than based on string sorting-order, so that
now the native architecture is parsed/displayed first. Observeable e.g.
in apt-get output.