Disable man-db settings to speed up package installation

The man-db process tends to be time consuming. By disabling the
auto-update + build-database settings, we can speed up the build
process and also package installation on the live system.

If someone really has needs for apropos(1) usage, manual invocation of
`mandb` should be all what's needed.

Thanks: Thorsten Glaser for the suggestion
Closes: https://github.com/grml/grml-live/issues/112