The split of the actual back-ends and usage of the alternatives
mechanism make it possible to have several back-ends installed on the
same host.

By the db backend has the highest priority. By this we ensure to stay
compatible while upgrading.

Only Tumbleweed users which used bogofilter after Apr 5th have to pay
some extra attention as their database might be sqlite3 driven. If
that's the case either:

a) zypper in -bogofilter-db bogofilter-sqlite3

or

b) switch the alternative to bogofilter-sqlite3

Thanks Martin for the alternatives implementation!

- Provide and obsolete bogofilter from the db subpackage to ensure we stay
with the same back-end in the update case.
- Require a backend package from the common package.
- Split of the documentation into an independent package.
- Tag the common and doc package as noarch post-11.1.

- Build different backends
* sqlite3 db and tokyocabinet
* switch between them using update-alternatives
- Disable checks as they do not work with suffixed builds