Remaining symlink issues

I now understand why symlinks are so painful in reprozip or CDE...

I think I successfully caught everything when packing, however issues remain when I have to select which files to unpack (`vagrant` without --use-chroot, and probably `directory`). In this mode, when a file is selected to be unpacked, the TAR archive has to be walked to find the links in its path, follow them, and list all the intermediate directories, intermediate links, and final target.

One other thing to consider is whether it wouldn't make sense for all these links to appear in the configuration file (and to be assigned to packages) as well.