Directories were ignored by mistake, we found out that several files
that would help us diagnosing problems were missing, because we only
snapshot etc/* which did not snapshot recursive directories such as
etc/promise or etc/service.

Symlinks were copied (the target was copied), because they matched the
isfile check, but if the symlink target was not existent, this caused
errors.

Also rename _snapshot_instance_file to _copySnapshot as it is no longer
treating files only