Add support for OpenBSD 5.6 and future, which apparently ships
unbound in the base system, and doesn't need to install a package
anymore.

In params.pp checking the operatingsystemrelease, to make the decision
whether to install the unbound package (version < 5.6) or not.

in init.pp, only install the package when package_name is defined,
and switch require statements into chained orders within that
same if statement.