unbound: refactor build options to select switches

The two unique packages "Unbound light" and "Unbound heavy"
were not working well due to the fact that Unbound is mostly
its library. Tools and helpers would crash. Instead a reasonable
default Unbound is built. Also up select options like python
are added. libevent and libpthreads are options to down select.

Signed-off-by: Eric Luehrsen <ericluehrsen@gmail.com>