MySQL 5.1 moved to Debian unstable

Thanks to Christian Hammers, we finally have MySQL 5.1 in Debian unstable, which completely replaces MySQL 5.0. Please update all build-dependencies from libmysqlclient15-dev to libmysqlclient-dev and upload a rebuild version of your package.

As a positive side-effect, we no longer care about MySQL bug #41728, which was the reason why no newer version of MySQL 5.0 made it into testing since lenny got released.