Fix for debian/ubuntu hold and a way to add debian src only

- modules "apt hold" handling is broken in debian stable cause files in /etc/apt/preferences.d will be silently ignored if files will be split by a blank

- add a way to add deb-src to sources.list (useful for build server like jenkins)

Sure. But then I don't understand what you're trying to do.

If you want only a `deb-src` line and no `deb` line it would have to be named differently, like `only_debsrc`, check if `include_src` is actually set to true and on a combination of those conditions only write out a `deb-src` line and make sure a `deb` line doesn't exist.

I also don't understand what your perceive to benefit from only having `deb-src` lines instead of having both `deb` and `deb-src`.