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)

correct me if im wrong

include_src is just an additional parameter to include a deb + a deb-src line to sources.list. its not possible to include a src line without an deb line using parameter include_src. right?