machine-image: extend search path

machine-image: extend search path

This adds directories in /etc and /run to the search paths for OS
images. While it doesn't make much sense to actually place huge disk
images there, it's good enough for symlinks to those.

The main reason for supporting this is that this allows us to neatly
symlink portable image files located outside of the search path into the
search path when attaching them, so that attaching them also means they
are discoverable properly for all commands.